I'm happy that you do!
The mimic randomly spawns between waves, like a regular chest. It shows signs of danger and soon I'll implement an intelligence check to detect if it's a mimic or not, but currently you have to spot it for yourself
If you press 'E' (Interact) on a chest, It'll open, but if it's a mimic you're in trouble...
Also, if you destroy a chest by damaging it, it will destroy the items inside, but the mimic will drop loot like a regular enemy. So the only way to loot a regular chest is by interacting with it.