How do you handle monster abilities like False Appearance? For example, Ropers are "indistinguishable from a normal cave formation" while motionless. Do you allow them to Hide as if they were heavily obscured? Are they automatically hidden? If so, what score do you use to determine whether someone's passive Perception detects them, especially regarding Surprise? That is, if there's a cave full of motionless Ropers, and the party walks in... is the party automatically Surprised (barring abilities that prevent being Surprised)? RAW says a contested Dexterity (Stealth) vs passive Wisdom (Perception) determines surprise... do you use that, using the Roper'sStealth, allowing it because even though they're in plain view, they're able to be "stealthy" because they don't look like monsters while motionless?
(Actually, while trying to figure out how best to describe my question, I pretty much convinced myself that that was the right way to go... allow them to roll Stealth even while not technically hidden, but I'll post it anyway, in case anybody has a better idea, a more official answer, some insight, or more appropriate quotes from the rules.)
There's no clear ruling on this. The only Sage Advice on this subject supports your interpretation, but not everyone agrees. The main question seems to be what "indistinguishable" means in this context.
Personally, I favor a middle-of-the-road approach. I have False Appearance grant the ability to hide in plain sight, with the creature still needing to roll a Dexterity (Stealth) check, but with advantage since their method of hiding is particularly effective. It seems to me that without granting advantage, the creature would be far too easy to spot on average, as most parties have at least one character with a high passive Perception.
I'm in a similar camp to jreggers: I add 10 to the stealth roll of the creature.
Is it possible to notice that it's a Dusk Mantle and not just a stalactite? Sure, but it's going to be nearly impossible, and I only give that leeway due to the fact that after your first encounter with a Dusk Mantle will teach you to watch for them. That +10 is going to make their stealth roll very difficult for newer adventurers, the more seasoned ones will be able to notice the minute details that could give away the creature.
"Indistinguishable from <some inconspicuous thing>" means exactly that: there's no way to distinguish the monster from whatever inconspicuous thing it's posing as. If the rules writer meant "nearly indistinguishable", they would've written that. There's no Perception check that'll tell you that's actually a monster. (Contrast that with a gelatinous cube, which is merely hard to spot.)
You determine surprise based on your understanding of whether the players identified a threat, same as in any other combat encounter. Did the walk past that roper without a clue? Did they touch a door not knowing it was a mimic? Surprised. If someone says "Let's approach carefully, that might be a roper over there...", then no surprise.
I think that's a fair position to argue based on the RAW. The problem I have with this approach is that for me, it defies the setting's internal logic. Imagine for a second that we lived in a world where some chairs were vicious monsters, but it was almost impossible to tell. Would you ever let your guard down around a chair again?
As a DM, I'm also not a fan of screwjobs, and a False Appearance ability that makes the creature literally indistinguishable reads to me like a screwjob unless used very carefully. It's no different than say, creating an undetectable booby trap, and I'm not sure what either of these is designed to accomplish. They're certainly not fun for the PCs.
But context definitely matters. If you treat False Appearance this way but you give the PCs some opportunity to realize that they should be on guard, then it's probably a non-issue. I prefer to err on the side of caution.
I think that's a fair position to argue based on the RAW. The problem I have with this approach is that for me, it defies the setting's internal logic. Imagine for a second that we lived in a world where some chairs were vicious monsters, but it was almost impossible to tell. Would you ever let your guard down around a chair again?
If every chair could be a mimic, probably not. But it's unlikely the chair in your house was replaced by a mimic, and even if you're exploring a dungeon, it's easy to get sloppy after the 20th door. I've seen it happen.
As a DM, I'm also not a fan of screwjobs, and a False Appearance ability that makes the creature literally indistinguishable reads to me like a screwjob unless used very carefully. It's no different than say, creating an undetectable booby trap, and I'm not sure what either of these is designed to accomplish. They're certainly not fun for the PCs.
Agreed. I'm not advocating using these monsters as a cheap sucker punch, just stating the facts: according to the Monster Manual, there's no way to tell a mimic from a regular object or a roper from a cave formation. That's part of what makes them dangerous and memorable.
There's many ways A DM or adventure could telegraph these monsters. They could call for knowledge checks when they enter the dungeon, have NPCs warn the party, leave signs of the monster's previous victims, populate the dungeon with related monsters (e.g. where there's ropers, there's usually piercers), or just straight up tell characters with certain backgrounds (e.g. drow, mountain dwarves, certain rangers...)
The way I think about it is that the party will have to use other reasources than just their perception and caution to get by. I have one player with a +10 to their perception, and they are the type who don't like surprises and find enjoyment in checking the 20th door thoroughly.
I guess in my head I see the party going down a corridor, checking for baddies, seeing stalagtites and stalagmites and saying "crap we have to be careful. ok so lets clear out the piercers and darkmantles" *casts shatter on ceiling* ok now the ground ones *casts dmg spell* "oh no it's a roper!" **engage combat**
I played them discoverable by checks and it turned out sub par for the experience of fighting creatures who should be able to blend in with the walls. Maybe I should just raise the stealth mod or roll adv like DMThac0 or jreggers.
I’d like to resurrect this thread, because it’s related to a distinct concern I have over an upcoming session. I’m using Grey Oozes and Violet Fungi in a cave, and I’ve been wondering exactly how to handle the “indistinguishable from ——“ trait. I kinda like the idea of adding a modifier to their stealth rolls, but I’m wondering if anybody else has insights or experience with this issue.
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I live with several severe autoimmune conditions. If I don’t get back to you right away, it’s probably because I’m not feeling well.
False Appearence has no effect on Stealth whatsoever. It doesn't hide your position or make you invisible and silent, it makes you indistinguishable from a normal item. In fact, for this feature to really have any meaningful impact in the game, it must be seen.
In my opinion, a stealth role is still needed. When they are still, they are indistinguishable. The stealth check measures their ability to hold completely still.
I'm not a fan of a stealth roll opposed by passive perception just because it requires nothing at all from the players. People suggest adding modifiers to stealth or giving them advantage for the simple reason that failing that stealth check is boring and makes the scene more about monster incompetence than anything else.
I'm not going to screw the party outright, but passive perception is not going to notice these creatures unless a PC has specifically put resources into it (like the Alert feat), and even then they would only notice moments before the attack.
What passive perception might do instead is notice signs as IC mentioned back in post #6. Mysterious remains, other creatures behaving strangely, strange marks on the walls or floor. Stuff that adds tension and encourages the players to act. Active perception checks might spot a stalactite that doesn't look quite right. Active investigation might deduce from the clues uncovered so far that something routinely uses this room to ambush prey. The rest I might leave to the players.
False Appearence has no effect on Stealth whatsoever. It doesn't hide your position or make you invisible and silent, it makes you indistinguishable from a normal item. In fact, for this feature to really have any meaningful impact in the game, it must be seen.
Exactly. It does make them stealthy, they probably won't even by trying to sneak. (Other than blending in)
When the player enter the area with a hidden monster I don't include the monster when I'm describing the area. When the players enter an area with a monster that has false appearance, I describe in the description so the players know it's there, but I describe it as it's false appearance.
- No stealth roll at all. They are exactly what they look like. - Instead, I add clues or warnings to their potential presence if the PCs are investigative. Eg. a corpse in the middle of an empty room, rotted tendrils lying around the room that had been cut before, etc. This encourages more dungeon-delving than just “hide + perception”. If the PCs do not investigate the rooms I describe to them, then they are usually always surprised by these things. - I do not use these creatures often, only as a small encounter/foil to a bigger encounter.
In my opinion, this becomes a significantly lower issue if you include the other senses.
So a roper is indistinguishable from a stalagmite when it doesn't move - so even if you look straight at it, you'll se a stalagmite. Your eyes won't help you here - it's almost like they were added because of the "I have darkvision!" thing!
So the roper rolls stealth and a characters passive perception beats it. That character might hear something breathing, or smell something. What they won't do is see the monster, because it's indistinguishable from the rock. But they might hear it, or otherwise sense it, and that might prompt them to make a perception check and determine which direction it's in.
So, with regard to using oozes and the like, I would have them roll stealth as normal, but a failure doesn't say "hey, that puddle is an ooze!" it says "you sense something is wrong - you smell some carrion-stink on the air, and a peculiar acidic tang, a metallic taste on your tongue". That should put them on edge and make them wary, and then the ooze doesn't necessarily get a surprise round, should combat ensue, and may even be discovered before it pounces.
Then, if they encounter a hidden ooze next time and you describe it the same, the players (if they are paying attention) will say "ooh, I remember this, that's an ooze!", and feel really good about themselves!
In my opinion, this becomes a significantly lower issue if you include the other senses.
So a roper is indistinguishable from a stalagmite when it doesn't move - so even if you look straight at it, you'll se a stalagmite. Your eyes won't help you here - it's almost like they were added because of the "I have darkvision!" thing!
So the roper rolls stealth and a characters passive perception beats it. That character might hear something breathing, or smell something. What they won't do is see the monster, because it's indistinguishable from the rock. But they might hear it, or otherwise sense it, and that might prompt them to make a perception check and determine which direction it's in.
So, with regard to using oozes and the like, I would have them roll stealth as normal, but a failure doesn't say "hey, that puddle is an ooze!" it says "you sense something is wrong - you smell some carrion-stink on the air, and a peculiar acidic tang, a metallic taste on your tongue". That should put them on edge and make them wary, and then the ooze doesn't necessarily get a surprise round, should combat ensue, and may even be discovered before it pounces.
Then, if they encounter a hidden ooze next time and you describe it the same, the players (if they are paying attention) will say "ooh, I remember this, that's an ooze!", and feel really good about themselves!
Okay, that sounds like a good compromise. Unfortunately this week’s game was cancelled because I was sick. So I’ve got another whole week to second-guess myself… it will give me plenty of time to write good descriptions, though!
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I live with several severe autoimmune conditions. If I don’t get back to you right away, it’s probably because I’m not feeling well.
So a roper is indistinguishable from a stalagmite when it doesn't move - so even if you look straight at it, you'll se a stalagmite. Your eyes won't help you here - it's almost like they were added because of the "I have darkvision!" thing!
So the roper rolls stealth and a characters passive perception beats it. That character might hear something breathing, or smell something. What they won't do is see the monster, because it's indistinguishable from the rock. But they might hear it, or otherwise sense it, and that might prompt them to make a perception check and determine which direction it's in.
This doesn't work for monsters like elementals (e.g. gargoyles) or animated/haunted/sentient objects. They're already made out of the exact same stuff as their inanimate counterparts would be. There's no respiratory, circulatory or nervous system that could cause involuntary movements. I stand by my first post: if you could tell them apart using one of your senses the writers wouldn't be using the word "indistinguishable" and considering this is a special rule, if there was a workaround it would be mentioned. Compare with something like Disguise Self that explicitly tells you other creatures can succeed on an Investigation check to find flaws in your disguise.
Also a Perception vs Stealth contest seems like a really weird choice to me. Even if ropers had an odor that'd give it away, it can't hide its own odor, so Stealth shouldn't be part of the equation. Noticing the smell should be a simple Perception check against a fixed DC.
False Appearance should probably allow a check of some sort, it's basically a trap that happens to be a monster (though how trap detection works is also... not as clear as it could be), but there's no reason it should key off of stealth.
So a roper is indistinguishable from a stalagmite when it doesn't move - so even if you look straight at it, you'll se a stalagmite. Your eyes won't help you here - it's almost like they were added because of the "I have darkvision!" thing!
So the roper rolls stealth and a characters passive perception beats it. That character might hear something breathing, or smell something. What they won't do is see the monster, because it's indistinguishable from the rock. But they might hear it, or otherwise sense it, and that might prompt them to make a perception check and determine which direction it's in.
This doesn't work for monsters like elementals (e.g. gargoyles) or animated/haunted/sentient objects. They're already made out of the exact same stuff as their inanimate counterparts would be. There's no respiratory, circulatory or nervous system that could cause involuntary movements. I stand by my first post: if you could tell them apart using one of your senses the writers wouldn't be using the word "indistinguishable" and considering this is a special rule, if there was a workaround it would be mentioned. Compare with something like Disguise Self that explicitly tells you other creatures can succeed on an Investigation check to find flaws in your disguise.
Also a Perception vs Stealth contest seems like a really weird choice to me. Even if ropers had an odor that'd give it away, it can't hide its own odor, so Stealth shouldn't be part of the equation. Noticing the smell should be a simple Perception check against a fixed DC.
I am loathe to use Perception at all on this one, especially when the text says “indistinguishable”. This, in my mind, negates all forms of perception and requires a check for more investigation or other clues as to its whereabouts. If we consider this more of a trap than a creature, there should be some good clues that these monsters will leave. This would also force the PCs to be more wary of their environment rather than “I took Observant and have high Wisdom, I’ll always be able to see enemies no matter what”.
I am in the camp that "indistinguishable" means exactly what it says. NONE of a characters senses are going to tell them ANYTHING that will let them identify random stalactite as a monster of whatever persuasion. It is INDISTINGUISHABLE from its natural equivalent. Folks are welcome to house rule (or read it) however they like but I play it that way because these creatures blend into their surroundings so well that the creature itself gives no clues as to its presence.
However, depending on the creature, there could be remains from their hunting, there could be markings, scrapes or other sorts of tracks that might indicate the presence of something (i.e. the tendrils of a roper that grabbed prey from the ground and carried it over to its mouth are quite likely to leave scuff marks depending on the circumstances), there could also be a particular smell - perhaps of rotting meat - again due to previous victims. The creature may be indistinguishable but there MAY be clues as to its presence if the characters are playing close attention. In addition, if worst comes to worst, the characters can spend a minute or two launching an arrow or a firebolt into each stalactite and seeing if there is any unexpected response.
There are lots of ways to handle the situation and I don't find it necessary to resort to stealth vs passive perception for these cases. (As an example, I have a rogue character with the observant feat, at level 17 their passive perception is 27 - and it could be higher ... any character with the observant feat, a 16 wisdom and expertise at level 5 (say a variant human observant rogue) - would have a passive perception of 10+6+3+5 = 24. Even with advantage - a roper has to roll a 20(+5 stealth) to avoid being noticed by this character if you use the stealth vs passive perception approach. Hard to call that "indistinguishable".
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How do you handle monster abilities like False Appearance? For example, Ropers are "indistinguishable from a normal cave formation" while motionless. Do you allow them to Hide as if they were heavily obscured? Are they automatically hidden? If so, what score do you use to determine whether someone's passive Perception detects them, especially regarding Surprise? That is, if there's a cave full of motionless Ropers, and the party walks in... is the party automatically Surprised (barring abilities that prevent being Surprised)? RAW says a contested Dexterity (Stealth) vs passive Wisdom (Perception) determines surprise... do you use that, using the Roper's Stealth, allowing it because even though they're in plain view, they're able to be "stealthy" because they don't look like monsters while motionless?
(Actually, while trying to figure out how best to describe my question, I pretty much convinced myself that that was the right way to go... allow them to roll Stealth even while not technically hidden, but I'll post it anyway, in case anybody has a better idea, a more official answer, some insight, or more appropriate quotes from the rules.)
There's no clear ruling on this. The only Sage Advice on this subject supports your interpretation, but not everyone agrees. The main question seems to be what "indistinguishable" means in this context.
Personally, I favor a middle-of-the-road approach. I have False Appearance grant the ability to hide in plain sight, with the creature still needing to roll a Dexterity (Stealth) check, but with advantage since their method of hiding is particularly effective. It seems to me that without granting advantage, the creature would be far too easy to spot on average, as most parties have at least one character with a high passive Perception.
I'm in a similar camp to jreggers: I add 10 to the stealth roll of the creature.
Is it possible to notice that it's a Dusk Mantle and not just a stalactite? Sure, but it's going to be nearly impossible, and I only give that leeway due to the fact that after your first encounter with a Dusk Mantle will teach you to watch for them. That +10 is going to make their stealth roll very difficult for newer adventurers, the more seasoned ones will be able to notice the minute details that could give away the creature.
Here's my 2 cents:
"Indistinguishable from <some inconspicuous thing>" means exactly that: there's no way to distinguish the monster from whatever inconspicuous thing it's posing as. If the rules writer meant "nearly indistinguishable", they would've written that. There's no Perception check that'll tell you that's actually a monster. (Contrast that with a gelatinous cube, which is merely hard to spot.)
You determine surprise based on your understanding of whether the players identified a threat, same as in any other combat encounter. Did the walk past that roper without a clue? Did they touch a door not knowing it was a mimic? Surprised. If someone says "Let's approach carefully, that might be a roper over there...", then no surprise.
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I think that's a fair position to argue based on the RAW. The problem I have with this approach is that for me, it defies the setting's internal logic. Imagine for a second that we lived in a world where some chairs were vicious monsters, but it was almost impossible to tell. Would you ever let your guard down around a chair again?
As a DM, I'm also not a fan of screwjobs, and a False Appearance ability that makes the creature literally indistinguishable reads to me like a screwjob unless used very carefully. It's no different than say, creating an undetectable booby trap, and I'm not sure what either of these is designed to accomplish. They're certainly not fun for the PCs.
But context definitely matters. If you treat False Appearance this way but you give the PCs some opportunity to realize that they should be on guard, then it's probably a non-issue. I prefer to err on the side of caution.
If every chair could be a mimic, probably not. But it's unlikely the chair in your house was replaced by a mimic, and even if you're exploring a dungeon, it's easy to get sloppy after the 20th door. I've seen it happen.
Agreed. I'm not advocating using these monsters as a cheap sucker punch, just stating the facts: according to the Monster Manual, there's no way to tell a mimic from a regular object or a roper from a cave formation. That's part of what makes them dangerous and memorable.
There's many ways A DM or adventure could telegraph these monsters. They could call for knowledge checks when they enter the dungeon, have NPCs warn the party, leave signs of the monster's previous victims, populate the dungeon with related monsters (e.g. where there's ropers, there's usually piercers), or just straight up tell characters with certain backgrounds (e.g. drow, mountain dwarves, certain rangers...)
The Forum Infestation (TM)
The way I think about it is that the party will have to use other reasources than just their perception and caution to get by. I have one player with a +10 to their perception, and they are the type who don't like surprises and find enjoyment in checking the 20th door thoroughly.
I guess in my head I see the party going down a corridor, checking for baddies, seeing stalagtites and stalagmites and saying "crap we have to be careful. ok so lets clear out the piercers and darkmantles" *casts shatter on ceiling* ok now the ground ones *casts dmg spell* "oh no it's a roper!" **engage combat**
I played them discoverable by checks and it turned out sub par for the experience of fighting creatures who should be able to blend in with the walls. Maybe I should just raise the stealth mod or roll adv like DMThac0 or jreggers.
Just thoghts.
I’d like to resurrect this thread, because it’s related to a distinct concern I have over an upcoming session. I’m using Grey Oozes and Violet Fungi in a cave, and I’ve been wondering exactly how to handle the “indistinguishable from ——“ trait. I kinda like the idea of adding a modifier to their stealth rolls, but I’m wondering if anybody else has insights or experience with this issue.
I live with several severe autoimmune conditions. If I don’t get back to you right away, it’s probably because I’m not feeling well.
False Appearence has no effect on Stealth whatsoever. It doesn't hide your position or make you invisible and silent, it makes you indistinguishable from a normal item. In fact, for this feature to really have any meaningful impact in the game, it must be seen.
In my opinion, a stealth role is still needed. When they are still, they are indistinguishable. The stealth check measures their ability to hold completely still.
Only spilt the party if you see something shiny.
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I'm not a fan of a stealth roll opposed by passive perception just because it requires nothing at all from the players. People suggest adding modifiers to stealth or giving them advantage for the simple reason that failing that stealth check is boring and makes the scene more about monster incompetence than anything else.
I'm not going to screw the party outright, but passive perception is not going to notice these creatures unless a PC has specifically put resources into it (like the Alert feat), and even then they would only notice moments before the attack.
What passive perception might do instead is notice signs as IC mentioned back in post #6. Mysterious remains, other creatures behaving strangely, strange marks on the walls or floor. Stuff that adds tension and encourages the players to act. Active perception checks might spot a stalactite that doesn't look quite right. Active investigation might deduce from the clues uncovered so far that something routinely uses this room to ambush prey. The rest I might leave to the players.
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(Artificer) Swordmage | Glasswright | (Barbarian) Path of the Savage Embrace
(Bard) College of Dance | (Fighter) Warlord | Cannoneer
(Monk) Way of the Elements | (Ranger) Blade Dancer
(Rogue) DaggerMaster | Inquisitor | (Sorcerer) Riftwalker | Spellfist
(Warlock) The Swarm
Exactly. It does make them stealthy, they probably won't even by trying to sneak. (Other than blending in)
When the player enter the area with a hidden monster I don't include the monster when I'm describing the area. When the players enter an area with a monster that has false appearance, I describe in the description so the players know it's there, but I describe it as it's false appearance.
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>Extended Signature<
I use “False Appearance” creatures this way:
- No stealth roll at all. They are exactly what they look like.
- Instead, I add clues or warnings to their potential presence if the PCs are investigative. Eg. a corpse in the middle of an empty room, rotted tendrils lying around the room that had been cut before, etc. This encourages more dungeon-delving than just “hide + perception”. If the PCs do not investigate the rooms I describe to them, then they are usually always surprised by these things.
- I do not use these creatures often, only as a small encounter/foil to a bigger encounter.
In my opinion, this becomes a significantly lower issue if you include the other senses.
So a roper is indistinguishable from a stalagmite when it doesn't move - so even if you look straight at it, you'll se a stalagmite. Your eyes won't help you here - it's almost like they were added because of the "I have darkvision!" thing!
So the roper rolls stealth and a characters passive perception beats it. That character might hear something breathing, or smell something. What they won't do is see the monster, because it's indistinguishable from the rock. But they might hear it, or otherwise sense it, and that might prompt them to make a perception check and determine which direction it's in.
So, with regard to using oozes and the like, I would have them roll stealth as normal, but a failure doesn't say "hey, that puddle is an ooze!" it says "you sense something is wrong - you smell some carrion-stink on the air, and a peculiar acidic tang, a metallic taste on your tongue". That should put them on edge and make them wary, and then the ooze doesn't necessarily get a surprise round, should combat ensue, and may even be discovered before it pounces.
Then, if they encounter a hidden ooze next time and you describe it the same, the players (if they are paying attention) will say "ooh, I remember this, that's an ooze!", and feel really good about themselves!
Make your Artificer work with any other class with 174 Multiclassing Feats for your Artificer Multiclass Character!
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Okay, that sounds like a good compromise. Unfortunately this week’s game was cancelled because I was sick. So I’ve got another whole week to second-guess myself… it will give me plenty of time to write good descriptions, though!
I live with several severe autoimmune conditions. If I don’t get back to you right away, it’s probably because I’m not feeling well.
This doesn't work for monsters like elementals (e.g. gargoyles) or animated/haunted/sentient objects. They're already made out of the exact same stuff as their inanimate counterparts would be. There's no respiratory, circulatory or nervous system that could cause involuntary movements. I stand by my first post: if you could tell them apart using one of your senses the writers wouldn't be using the word "indistinguishable" and considering this is a special rule, if there was a workaround it would be mentioned. Compare with something like Disguise Self that explicitly tells you other creatures can succeed on an Investigation check to find flaws in your disguise.
Also a Perception vs Stealth contest seems like a really weird choice to me. Even if ropers had an odor that'd give it away, it can't hide its own odor, so Stealth shouldn't be part of the equation. Noticing the smell should be a simple Perception check against a fixed DC.
The Forum Infestation (TM)
False Appearance should probably allow a check of some sort, it's basically a trap that happens to be a monster (though how trap detection works is also... not as clear as it could be), but there's no reason it should key off of stealth.
…oh dear…
I live with several severe autoimmune conditions. If I don’t get back to you right away, it’s probably because I’m not feeling well.
I am loathe to use Perception at all on this one, especially when the text says “indistinguishable”. This, in my mind, negates all forms of perception and requires a check for more investigation or other clues as to its whereabouts. If we consider this more of a trap than a creature, there should be some good clues that these monsters will leave. This would also force the PCs to be more wary of their environment rather than “I took Observant and have high Wisdom, I’ll always be able to see enemies no matter what”.
Obviously, opinions vary greatly. :)
I am in the camp that "indistinguishable" means exactly what it says. NONE of a characters senses are going to tell them ANYTHING that will let them identify random stalactite as a monster of whatever persuasion. It is INDISTINGUISHABLE from its natural equivalent. Folks are welcome to house rule (or read it) however they like but I play it that way because these creatures blend into their surroundings so well that the creature itself gives no clues as to its presence.
However, depending on the creature, there could be remains from their hunting, there could be markings, scrapes or other sorts of tracks that might indicate the presence of something (i.e. the tendrils of a roper that grabbed prey from the ground and carried it over to its mouth are quite likely to leave scuff marks depending on the circumstances), there could also be a particular smell - perhaps of rotting meat - again due to previous victims. The creature may be indistinguishable but there MAY be clues as to its presence if the characters are playing close attention. In addition, if worst comes to worst, the characters can spend a minute or two launching an arrow or a firebolt into each stalactite and seeing if there is any unexpected response.
There are lots of ways to handle the situation and I don't find it necessary to resort to stealth vs passive perception for these cases. (As an example, I have a rogue character with the observant feat, at level 17 their passive perception is 27 - and it could be higher ... any character with the observant feat, a 16 wisdom and expertise at level 5 (say a variant human observant rogue) - would have a passive perception of 10+6+3+5 = 24. Even with advantage - a roper has to roll a 20(+5 stealth) to avoid being noticed by this character if you use the stealth vs passive perception approach. Hard to call that "indistinguishable".