Crimson Longinus
Legend
They suspect it to be a mimic. They won't be surprised that it is a mimic, that's what they expect!According to the OP, because they're "suspicious the chest might be a mimic". If they had noticed the mimic, they would know it's a mimic.
They have noticed a thing they suspect to be a threat.You don't have to know things are a threat. You just have to notice them.
Seriously, your ruling completely disregards the actual fictional situation, the player intent and the character attitude. It is, frankly, a terrible ruling, and this sort of thing would result the players simply not engaging with the environment or trying to do anything clever, assuming they would continue to play, which I wouldn't.