I did check! It went something like:
Me: Before I go further into this room, I'm backing up to the wall and shooting everything to see if any of it bleeds.
Party: You sure you're not going to get eaten by the wall?
Me: ...well if the wall is a mimic I'm already dead.
Evil DM: the bookcase is also a mimic.
I did check! It went something like: Me: Before I go further into this room, I'm backing up to the wall and shooting everything to see if any of it bleeds. Party: You sure you're not going to get eaten by the wall? Me: ...well if the wall is a mimic I'm already dead.
Alternately: The chest wasn't a mimic. The arrow was.