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There's a mistake in the stat block - metal doesn't have a smell. Therefore a rust monster cannot detect metal by scent (mostly).

The "metal smell" you get when touching a metal object is actually the oils in your skin reacting with the metal. To smell (untouched) metal, particles of it would need to somehow break off and become airborne to be detected by your nose, which doesn't really happen. The product of skin oils + metal is smellable, so the rust monster could find metal with that, but theoretically if you didn't touch any of your metal items you could sneak past, assuming that the rust monster doesn't see it. I don't know how this would be useful for the monster, because >90% of all ferrous metals it walks past will not be carried by a helpful adventurer to make it smelly.

If you had liquid metal for some reason, it might be smellable, but I haven't done any research on that.

I learned all this from an interesting YouTube video from NileRed, he goes into way more detail and actually makes the metal-smell compound, https://www.youtube.com/watch?v=BqLH-nTZEOc

I liked this as a "more scientific" way of explaining rust monsters' metal detection ability.

Quote from Bellsange >> 

I don't know of an explanation in the lore, but... They locate their desired metal by detecting magnetic fields, which are produced by ferrous metals. 

there has been a level 2 warforged paladin with 20 ac making my DM life a living hell. the gods have blessed me with a solution

Baby gloves edition of the rust monster is boring just like the rest of baby gloves edition.

Judging by their size in the Honour Among Thieves film, they should be small, or even tiny

So basically a pet metal detector?

steel is made from iron, and this thing eats iron, preferably, so I would not advise anyone attacking a rust monster with those metals.  Bronze would be good for killing one. other nonferrous metals could be used. An elven druid, with glassteel weapons and nonmetal armor, keeping one of these as a pet, could reap some havoc. well, this beast is not classified as a magical creature. So it has a natural origin in the D&D world. The why, would be curious indeed. that having been said, dwarves would do well to kill these quickly. good luck with all those steel weapons. Try magic missiles. oh wait, dwarves do not trust magic users... To the best of my knowledge, precious metals are not ferrous.

love the desing

Quote from Uncharted_Lion >>

what does it do with magical weapons/items?

 Nothing, I assume. Magical items are far more resilient than mundane materials

Real cool i thing its one of my favorite corrector but not as cool as mimic but i like it its pretty cool i thing what about you gies???

when I used to play, my DM threw these at my fighter repeatedly. I had to buy a new longsword, shield and set of plate armour 4 times. With my in-game little sister constantly harassing me to get her something.

Ugh. 

Quote from LegendOfZub >>

Quote from Trewill91 >>

The most refined type of iron is just iron.  steel is iron with impurities (a very small amount of carbon)

 

Not quite. Pure iron is just iron, but steel is relatively pure iron. Basically as pure as they could get it back in the day. Carbon was (and is typically) used to extract the impurities. The carbon present in mild steels is often just remnants of the purification process. Pure iron is softer than pretty much any kind of steel, but most/all steel is softer than what most people would consider iron. You can add more carbon to make it harder for various tool steels, and you can add other metals to create alloy steels (I consider stainless steels to be in this category).

Steel: 0.002%-2% carbon by weight
Cast iron: 2-4% carbon by weight
Pig iron: 4-5% carbon by weight

 so it's probably more like how we put sauces on food and adding too much ruins the taste

Quote from TurtleLord123 >>

Quote from Shinjithegale >>

What about silvered weapons?

 A bit late, but here is an answer:
Silver is not a ferrous metal and doesn't rust, so the rust monster couldn't remove the "silvered" property from the weapon. It would still take the -1 penalty though, because the weapon is probably still mostly iron/steel (silver is really soft so a fully silver weapon would suck).

Quote from TurtleLord123 >>

Quote from Shinjithegale >>

What about silvered weapons?

 A bit late, but here is an answer:
Silver is not a ferrous metal and doesn't rust, so the rust monster couldn't remove the "silvered" property from the weapon. It would still take the -1 penalty though, because the weapon is probably still mostly iron/steel (silver is really soft so a fully silver weapon would suck).

 little late too so what about other metals that rust like copper

wait why dose it still say medium for the size?

Quote from Reuben_Medik >>

Judging by their size in the Honour Among Thieves film, they should be small, or even tiny

Best way to use these creatures is to have them as campsite menaces. Wait till the party is sleeping, then whoever is on watch is alerted to the sound of scraping metal. Shining a torch toward the sound reveals a rust monster eagerly feasting on the empty-sitting armor of the party's fighter or paladin, merrily munching away while the party sleeps. This image is very reminiscent of shining a flashlight to discover a raccoon going through your garbage. It's a nuisance monster, but one that can change your game plan next day, as most adventurers don't carry an extra set of heavy armor with them. 

Okay, so there's some verbiage about ferrous vs non-ferrous, but nothing about inert, "untarnishable" metals like gold and platinum. Also, aluminum isn't ferrous, but certainly corroded.

Silver tarnishes (a.k.a, rusts).

Does the rust metal trait affect magic items if they are within an antimagic field? It says in the spell that magic items in the field function as if they are nonmagical

Adorable

As a newer player, I made a ranger tiefling and was wondering if it might be possible to have one as a familiar/pet?