Blow-someone-s-cover

blow someone’s cover

To ruin or nullify one’s disguise or assumed role; reveal one’s true identity: The undercover cop had to blow his cover by pulling his gun when he thought they might have spotted him/ She called out my name, which blew my cover
To reveal something, esp inadvertently or mischievously: I’m not blowing the movie’s cover story by giving you this information (1970s+ Espionage & police)