Don't need anything as clumsy as a drone swarm. There are systems designed to aerially record an entire city at high enough resolution to follow each individual. And to keep history long enough to go back to before you thought you had to be careful.
I was extrapolating a lower cost version of Gorgon Stare. I remember when Rivet Joint first came on line and it was possible to track Warsaw Pact units as they traveled, and you could extrapolate their C3 units as well as their POL supplies. Never thought it would become part of modern police surveillance, but I was a young naive person back then.
