It's very difficult to operate a moderate-sized event secretly. Raves used to be promoted locally (at record & clothing shops) , and then shared among friends. The venue was secret right up to the event, because you first had to drive to a number of secret waypoints that were revealed by phone. The final location was often hidden in the wilderness or at a condemned (or squatted) building.

Accidental and deliberate surveillance is so common and cheap that this is no longer viable.