Say 1 in every 1,000 chests is a deadly mimic. And when I say deadly, I mean *deadly*, enough to give ppl pause in opening chests. Any chest could be a mimic, even raid or epics.
The mimic is boss level CR for its dungeon, and drops up to +5 depending on the difficulty. A battered and unprepared raid party wiped from a mimic end chest would be a funny sight to see.