Aircraft circling low, with white lights flashing

I don't know if military planes have ADS-B transmitters, or some other system, or maybe they're allowed to turn them on/off as required?

apparently they do.

but they can switch from "Mode S" to "Mode 5" when they don't want to be tracked.

Flying VFR, you have to keep a lookout for other aircraft. That is much easier when orbiting (flying in a circle) than when trying to fly a racetrack. You can orbit purely by using external references. A racetrack hold needs you to spend a lot of time looking inside the cockpit at your instruments and at the time. Not a problem when flying IFR, as ATC radar should keep you safe from other aircraft. A serious problem when trying to keep a lookout for other aircraft.

As a private pilot, I only had to meet my own standards. :-)

There was a newspaper (forgot which one) story recently about the police in Scotland somewhere have bought a few planes and then found them unsuitable for what they wanted. They were trying to sell them (cheap) to some UK forces. Maybe this was a trial by prospective buyers.

We see quite a few here, and normally it's turned on.

Andy

I built one of the coke-can antennas for a DVB-T based SDR, it picked up ADS-B transponders ok ... that's basically how all the flightradar sites get their data.