In the dungeon of a moderately sharp necromancer, my players encountered a treasure chest in the first room. A stereotypical treasure chest in every way from the outside. So stereotypical they shot it with a crossbow from the other side of the room because what if it’s a mimic.
Next session they found another treasure chest, deeper in the dungeon. The halfling gave it a cursory glance, walked over to it and tried to open it without checking for mimicry at all. As in the halfling player, Robin, said “I don’t even check”. It was a mimic.
Eric asked if it had always been there or if I added it after the previous session. It had always been there. This dungeon is large and has been fun, as I drew out most of it on a plane trip across country months ago and I can only barely remember what some of the bits even are any more. The takeaway, though, is that if a mimic takes up residence in your dungeon, you should definitely put a decoy completely real treasure chest in the first room, it’s super effective.