Not a pilot, but played with ADS-B tracking several years ago, and my impression was below something like 4-5000' the transponder wasn't required to transmit location. This meant that surveillance aircraft (usually single engine propeller craft and helicopters) didn't send GPS, only the fact that it was in the air.

Yeah. It's a requirement in a lot of US airspace, but not all airspace. Even at low altitudes it's required near major airports though.

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.)

If turning ADSB off is only allowed for government / military / intelligence airplanes, would using a CV algorithm to capture these airplanes be against the law?

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...

I don't see a B2 at your link?

Oh wow, they removed it. I apologize, I simply shared the link that had been shared before in December 2021. I should also have taken a screenshot.

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...

In fact, ADS-B E has an option to filter only aircraft not shown on other sites :)