As someone else suggested, think of it like parallel construction in law enforcement: an agency already knows someone is guilty, but they know it from illegal surveillance they can't use in court. So they give a tip to an unsuspecting officer to watch out for a car with a broken tail light at a particular location/time, and in the course of the traffic stop the officer finds drugs in the car, then they claim that's how they found out (despite the officer only being there because they already knew).