"Secret surveillance" refers to the act of monitoring or watching someone or something without their knowledge or consent. It means observing or spying on someone or something in a hidden or covert manner. Full definition
Why else would the revenge story stop in its tracks to swap the team over to an unrelated story involving a top - secret surveillance system? (markreviewsmovies.com)
By going behind the curtain of American secret surveillance, we're indulging in our collective curiosity at how exactly all these moving parts operate. (rottentomatoes.com)
Kelly as commissioner defended a police operation, exposed by The Associated Press, that conducted secret surveillance of Muslims. (fortune.com)