There was a website shown on HN a few years ago that used AI and plane transponder data to find circling planes which were presumably doing this kind of surveillance over American cities. It might have used further parameters to narrow it down, e.g. “over a city, circling for >3 hours” to rule out planes waiting to land. I thought it was named something simple like “plane-circles.com” but I’m not having any luck finding it again.

See also https://en.m.wikipedia.org/wiki/ARGUS-IS

Edit: found it. Should have limited the search to HN from the start. https://news.ycombinator.com/item?id=24188661