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...