People often think an aircraft is flying with transponder off because it doesn't appear on FR24/FlightAware. In fact these companies allow blocking aircraft from their service; this is why ADS-B Exchange is better.
Surveillance aircraft, if operated by a government entity, would not be required to transmit anyway. (Though they may choose to for safety purposes if it doesn't compromise the mission, just as the military often do in busy airspace.)
Recently there was a post of Google satellite cameras capturing a B-2 Stealth Bomber showing up on Google Maps [1]. Would such planes be captured by a vision system only, and would that be of concern to the government that might be trying to keep it undetected?
1: https://www.google.com/maps/place/39%C2%B001%2718.5%22N+93%C...
Here is the HN post: https://news.ycombinator.com/item?id=29627105
Here's a Techradar post on this: https://www.techradar.com/news/eagle-eyed-redditor-spots-a-f...
I'm sure some people don't like the noise, but I love seeing the planes so close.
A number of planes fly low and when I check flightaware, they're not on it. Or there's a plane that could be it, but it's flight path is heading East to West 30 miles North of me, while this plane came from the South heading North. Also, pretty much once a month or so, a pair of A10 Warthogs fly over or near our house. Last month, we had a real treat with 4 of them directly over the house, better than an air show.
So I'd like to setup 4 cameras on my tower focused skyward and have them try to catch clips of planes. I'll be looking at your work to see if I can also match those with flightaware.