I'm going ahead and hit "doubt" on that statement as I looked at my 2014 Monster Manual before writing my previous message and the word "nearly" is absent in that description.

I don't know what to tell you. I went to 5etools, looked at the 2014 lore, and directly copy-pasted that exact quote. You can check yourself. If I wrote it, I'd have spelt unrecognisable with an s instead of a z. Maybe it got errata'd at some point?

Ok, so I'm sorry about my previous tone, it seems that the mimic article, on 5etools and the SRD website, is the source of our confusion and disagreement: each time the description appears twice, first without the word "nearly" (under "False Appearance") then once with it. That 2nd description, under "Imitative Predators", does not appear in the Monster Manual. I could not check what D&D Beyond says because I do not have access to its contents.

Yeah, I think 5e tools uses the first ever printed version, while WotC reprint and edit the lore in the Monster Manual a LOT. D&D Beyond would probably be a third entry entirely. I'm glad we're on the same page now (or rather, we were on the same page, but the books were different).

Motionless and _indistinguishable_. Chests and crates don’t breathe and you’d be able to _distinguish_ the two very easily based on that. An _investigation_ check could work, or maybe a straight intelligence roll. Paying attention to the description of the room, too, and passive investigation is a real thing as well. I’ve already explained at least twice that you can use those ways to figure out that something isn’t where you left it or otherwise seems out of place. You can absolutely still find it using mechanisms that don’t require getting chomped. You’re just wrong. It’s fine, it happens, but the plain english is making it reeeeeally hard for me to understand how on earth you could be confused here.

Indistinguishable *if motionless*. If not motionless, distinguishable. You seem to be assuming that because it CAN be indistinguishable, it IS indistinguishable, and thus cannot be moving. Meanwhile, Dark Souls has mimics that breathe, and they work perfectly fine. Easy to get caught out, but definitely possible to spot if you're careful. Yes, you have explained twice how you can distinguish a creature that cannot be distinguished. And I've pointed out how paradoxical that is. Like, are you okay? Genuinely, I'm getting concerned for you.

Didn't know that, actually. How's their poison resistance? Can they hold their breath? Does D&D have spells that let you do things like deoxygenate a room?

Holy shit you’re dense. This isn’t Dark Souls and that’s a video game, not a table-top, theatre of the mind, RPG. If you can learn that the crate, _which is indistinguishable from any other crate_, has no reason to be where it is then you can perform further tests to expose whether or not it is a mimic. It will look exactly like a crate the whole time and that’s fine. It’s not a paradox, you’re just not very intelligent. You’re fifty-two cards short of a deck and making it everyone else’s problem.

They're immune to acid but not poison. They have a +2 CON mod, so they can hold their breath for 3 minutes, then have 2 rounds to breathe again or they die. It's rare for a room to be air-tight in D&D, but if it was, a simple "create bonfire" would do the job. Also, a mimic dying from smoke inhilation would be awesome.

No idea. Not even the most meta-gaming-est members of that party had a workaround.