blow (one's) cover
To expose one's true identity or motives after they had been intentionally concealed (i.e. after one had been "undercover"). The spy fled when he realized that someone had blown his cover. I totally blew my cover when I stupidly wore my school jacket to steal the other team's mascot.
blow someone's cover
Sl. to reveal someone's true identity; to ruin someone's scheme for concealment. The dog recognized me and blew my cover. I didn't mean to blow your cover by calling out to you.