An actual exchange in a group my wife was in, back before we were even dating:

Player: "I wanna catch the stone from his sling! What should I roll for that?"

DM: "Damage."

I get the joke, but with no prior context it's kind of a jerk move to assume the player already opened it if they asked for a perception OR an investigation. A Mimic would normally just ambush you OR attack when the player opens them. If it was opened it's probably already attacking, and if it was closed then performing a perception shouldn't cause initiative until rolled (if the mimic noticed you noticing it, for instance).

I'll....I'll crawl back into my dingy nerd tavern now...