This has been occurring intermittently for many years in the northern Virginia suburbs of Washington, DC - a small, high-wing plane flies precise circles for hours and hours over a particular area, which varies. Each loop takes about five minutes. In the past, the plane could be tracked, and was found to be based at the Manassas, VA airport. But in the past few years, it apparently has its ADS-B and Mode S turned off - it never shows up on this or other tracking sites. Years ago, scanner listeners heard it using a ROSSxxx callsign, which I understand is associated with the FBI. To give you an idea how long the flights have been going on, there was a story in the 8/28/1987 issue of the Washington Post, which noted that residents of Fairfax City were curious about the flights, but were only told it belongs to a government agency, and that no further information would be provided.
Primarily during the day, but occasionally (like yesterday) a flight continues into the evening. They fly pretty high, and are hard to spot unless the weather is very clear. The sound, however, is quite noticeable if you're paying attention and it's otherwise quiet. Being in the DC ADIZ, we don't see many small planes - most of the traffic is airliners and helicopters. Interestingly, yesterday afternoon there were two planes circling - the first time I'd seen that. The circles were adjacent to each other - not overlapping - and flown in opposite directions. Given that the flights are so frequent and have been going on for so long, I'm inclined to think that some or most of them might be training missions from the FBI Academy in Quantico.
