An undercover agent is a person who works secretly, pretending to be someone else in order to gather information or catch criminals without them knowing. Full definition
His mission: track down and retrieve sensitive information from a fellow undercover agent who has gone dark. (gaming-age.com)
On several occasions, he met up with undercover agents posing as customers where he accepted cash payments for electronic bitcoin transfers. (cryptotaxprep.com)
We're the worst undercover agents in movie history! (blog.bullz-eye.com)