My brother got paced and then Ka'd by a Utah Highway patrol chopper on some long desolate stretch of Utah highway. His 8500 picked up an increasing Ka signal and he couldn't see any cops around. Luckily, his granddaughter in the backseat said "A helicopter is following you." So they shoot Ka band and fly low behind so you don't see them coming. They flew ahead of him and turned around and that's when he discovered it was a Utah Highway Patrol chopper.
Got to thinking, since we are both pilots, about using the transponder which is required in all aircraft by law to transmit to the ground-based radar units at the airports and centers. Maybe a way to pick up that signal and determine how far a police aircraft or chopper is from you?
Seems someone has already done it: Monroy ATD-200 | Collision Avoidance
They also have one for $100 more that reports distance and altitude on a display verses the simpler LED bargraph of the ATD-200. Both work out to a distance of 5nm and could be handy on those long desolate stretches like I-5 in California where the CHP mostly resides near communities and uses air cover and pacing/Vascar.
Now if they would identify the "Squawk frequency" of the aircraft's transponder it would identify who the cops are verses any other type of aircraft. I'm guessing they fly with a set transponder squawk code so air traffic control can easily identify them.