New DM here

Last session I left my players in a room halfway through a dungeon. The only exit is the one they came through, but there's also a secret door in the room that they haven't discovered. To reach the other half of the dungeon they need to go through the secret door, but as far as they know they might be done and try to leave the dungeon.

What should I do to help them discover the rest of the dungeon?

While they're in the room, ask one of your players to make a perception check instead of waiting for them to ask you. On a success, you might give a hint like "you notice a set of footprints walking towards this wall before vanishing next to it." If they fail, the other players might guess there's something there to look into. It's a little meta-gamey, but worth it when teaching new players what kind of things to watch out for.

Yeah, definitely feel free to force a perception or investigation roll (or both!). 

Also, remember passive perception/investigation - maybe a character will just notice it without a roll? Also, doesn't just have to be sight, but could notice via smell, hearing, or touch (i.e. breeze or something).
Or, maybe put some sort of trap in there that might give the idea that there's something important about this room?

Or, you could move the door to a different area as they leave, make it not secret, and it can be like a magically moving door, if you definitely want them to explore the rest of the dungeon.

Never hide something integral to the story, or something you want them to find, behind a roll of the dice.

If the players need to find something that is plot relevant you should put a different obstacle in their path. Make it combat centric, where a bad guy uses the hidden door as an escape path. Have notes, journals, and architectural blueprints laying about that talk about a hidden door. Have the hidden door leave scuff marks on the floor, disturb dust in the room from a small draft, or even be slightly ajar. Something that you can use to give them information that there is a hidden door to find.

Always leave yourself some wiggle room to adjust the dungeon/story to your players' decisions. However, if it is not integral that they find the hidden thing...then they miss it. They won't know they missed anything, it won't harm them or their fun, and if you want to bring them back to the location, you can just make a new hook to send them back.

Yes, I might be able to start the next session by asking for a roll and either give them some hints or make them suspicious enough to actively search, seems like a good plan.

Passive perception, traps, moving or revealing the door wouldn't work in this situation. Last session they went into the room, fought some zombies there and then we ended it. It was also the last room on this side of the door so I can't move the door to another room.

It's not terribly important that they find the rest of the dungeon, it's not like the plot depends on it or anything. It's mostly just to get more out of the dungeon, let them fight some monsters and find some loot without me having to come up with more plot hooks and so on. The dungeon might also feel a little anticlimactic if it is only half the intended size.

That's part of the risk/reward of dungeons though. 

I have run many dungeons, one of the best examples is Sunless Citadel. There is a door near the beginning of the game that has a special need before it opens. It's possible to bypass this with high enough checks, but it had never happened in the dozens of times I've run the game. Every group that I had run decided to forgo opening the door by the time they got the necessary item to open it. Behind this door are some nice rewards and an interesting story bit. The players didn't feel any loss from it, even though they knew that it existed. 

This is a case where you, as the DM, feel they should be able to go further in the dungeon. Have the players said they're let down by the dungeon? Then give them the answer to the door. Are the players just fine moving on and finding the next adventure? Then let them have the miss, it's not integral and they're none the wiser. They're in the driver's seat, your story adapts to their choices. 

Passive perception could easily work though. If one of the PCs has a passive high enough to hit the door's DC all you have to do is say "Player X, as you're looking around the room you notice some marks in the dust in front of one bookcase - it looks like the bookcase has moved recently." Or something along those lines.

If worse comes to worst, you can always say "Player X, you suddenly notice a door on the far side of the room. It's like stupidly obvious, and you have no idea why you didn't see it before." :D

I mean, in this case it is less of "my players didn't interact with the things I prepared for them" and more of "I failed to give my players access to the things I prepared". The players are in the driver's seat, but it is my job as a DM to provide roads, road signs and destinations. If the players are following a boring road that doesn't lead anywhere or miss the crossroads (not sure that is the right term but whatever) to the interesting sites, then I have failed.

But yes, I do see your point, and ultimately, this particular dungeon does not really matter.

That's fair, boring is boring, and we are always trying to do our best to give them every opportunity to do exciting things. Mistakes happen, embrace them and laugh. Going forward you have a better idea on how you'll approach this type of situation. 

Quote from DMThac0 >>

. 

I have run many dungeons, one of the best examples is Sunless Citadel. There is a door near the beginning of the game that has a special need before it opens. It's possible to bypass this with high enough checks, but it had never happened in the dozens of times I've run the game. Every group that I had run decided to forgo opening the door by the time they got the necessary item to open it. Behind this door are some nice rewards and an interesting story bit. The players didn't feel any loss from it, even though they knew that it existed. 

 

 That is the same with my parties. I find it wierd and interesting that they do X to get the thing and then just leave.

think about how often this dungeon is used. Is this trap door used regularly? if so there would likely be wear near and around any mechanisms. if not, then think about who put the trap door there, do they want it to be discovered? or is it meant to stay hidden? if its meant to stay hidden, think about whats been done to hide it. is it covered? if so have the party role investigation when they look around the room.  if all else fails, when they turn to leave say "you feel a faint draft coming from inside the room"

I'm firmly in the the Players don't need to find everything camp. 

• The adventure shouldn't be dependent on them finding the secret door - unless you're fine with the Adventure failing at that point ( and that's a possibility if it happens in a narratively logical and satisfying manner, and allows them to springboard to new adventures ). 
• The dungeon shouldn't be boring without the content on the other side of the door. It can be more interesting if they find it, but they still should be able to have a good time if they don't.
• As a DM you don't lose anything, as you can now just take the content behind the door and put it in the Stuff I Prepared but Didn't Use box, to be pulled out and recycled later. After all, the Players will never know that you're recycling unused material :)

I think it is important that Players/Character have the chance to lose, or at least not do as well as they possibly could have. This doesn't mean that Players have to be miserable, or not have a good time ( that's just being a bad DM ), but having chances to fail at their objective, or miss out on material, makes their victories actually mean something: If you can't ever lose, then winning isn't that special.

Quote from DMThac0 >>

Never hide something integral to the story, or something you want them to find, behind a roll of the dice.

Vedexent That might be good general advice to keep in mind for next time, so thanks for that. Doesn't help in this context though, since the question is what the best solution to an already existing problem is, not how to avoid it.

I don't quite get the "save unused material for later" thing, like yes obviously try to do that if you can, but it requires that a) the material is good and detailed enough for it not to be better to just make new material from scratch (unless for unexpected needs of course), and b) that it is flexible enough that it can be used under different circumstances, such as the concept of the dungeon or whatever, the level of the pcs, the group, etc. 

Saving unused material for later is kind of an automatic thing. I try to plan for what the party might do, but the party always surprises me. If I have a neat NPC or a location built and it never gets used, I file it away to wit for an opportunity to use it in the future--often with modifications for another situation. Very little of the creative material I use just gets abandoned entirely.

There are tons of ways to give clues to the presence of something hidden.

1) You give it to them. Someone in the party notices indications of the door. An unexpected straight line on the wall, Some mark on the wall or floor clearly indicating a door.

2) You give them clues that something is there. A draft or other movement of air, a dank smell in a dry dungeon, torch flames flickering due to moving air when it shouldn't be there, some other visual, audible or olfactory clue indicating something is off, maybe an unexpected echo or hollow sound from behind the hidden door? Any of these should get the party looking closer at their surroundings.

3) If they decide to rest there then the odds of noticing go up substantially.

4) Some creatures could try to ambush them from behind the hidden door. An insect/ants could come to raid their food crawling through a crack at the bottom of the door. Water could seep under the door or the remains of a puddle could be visible if there is occasionally water on the other side of the door.

If you want the characters to play the content then you just need a logical way for them to notice the door whether it is a clue if you want to play it a bit or just narrate that someone notices a likely hidden door.

-------------

In addition, as mentioned above, don't put content that you want the players to play behind a skill check of any kind. It is surprising how many times the entire party can roll under 5 for a perception check and miss seeing the DC10 door/switch/hidden thing that drives the next part of the plot. When they fail, you are stuck having to give something to the party when the game is much smoother if it is just narrated.

Remember, the players only know what you have told them, they don't know about hidden rooms, cool treasure or awesome encounters. As a DM you need to avoid getting too invested in your creation, the players don't know what is going on and never will so from their point of view, if you don't narrate it, mention it or otherwise include it, it never existed. This is also why, as a DM you should NEVER say "Wow, you missed some cool stuff when you failed that roll" since they never knew it existed and will be at best a bit annoyed, disappointed or irritated rather than walking away with whatever they did get feeling good about themselves.

If there is content you want them to play, items you want them to find, areas you want them to explore, then never gate any of it through a single skill check.