Running without ADS-B around major airports would be unacceptable. I think drones are going to be required to have them before 2020 which makes me think that isn't quite right.
ADS-B reduces controller workload as they don't have to manually tag individual aircraft, especially in a high traffic area. ADS-B also works without secondary radar interrogation.
When there is a high speed chase near an airport, sometimes the tower controller will request the helicopter remain under a certain altitude or clear of a specific area.
Additionally, websites like FlightAware.com have limited direct ADS-B coverage and also display the FAA 5-minute delayed feed. I've never heard of a "criminal" in the US using an ADS-B receiver to evade police helicopters in real time.
[0] https://en.wikipedia.org/wiki/Aviation_transponder_interroga...
[1] https://en.wikipedia.org/wiki/Traffic_collision_avoidance_sy...
[1] https://flightaware.com/news/article/Realtime-Flight-Status-...
Mode-C and Mode-S responses are sent when interrogated by secondary radar. ADS-B transponders with "Extended Squitter" transmit regularly while active, so no need to transmit your own interrogation pulses.
Its possible to monitor Secondary Radar pulses, and Mode C returns. It would require multiple geographically separated receivers to triangulate. I don't think FlightAware supports Mode C, only Mode S(MLAT) and ADS-B.
If you transmitted your own "secondary radar" pulses, the FAA and/or the FCC would probably launch a criminal investigation, with the FBI involved. Making your own "secondary radar" would be pointless and high risk.