Shotspotter, Are surveillance microphones legal without a warrant? HD

Expectation of privacy, being recorded without permission is a violation of my rights. Shotspotter claims to have the right to record and store your conversations,without your knowledge, for an unknown amount of time, listened to by who knows, and to be evaluated for any reason, not to be share with us. Notice how close they are placed to single family housing. Did these people sign a waiver to be recorded from their front yard or porch.?