Even privatebjets emit a transponder signal. I wonder how hard it would be to sniff this out of the air. Maybe some of our HAM friends would know.

It's called ADS-B, https://en.wikipedia.org/wiki/Automatic_Dependent_Surveillance%E2%80%93Broadcast

There are websites that will let you view the data across the world. You can use a cheap software defined radio and open source software to listen to and decode local traffic.

More to the topic are websites such as Elonjet which, tracks jets owned or associated with Elon Musk: https://grndcntrl.net/falconlanding This is all done with public information (Elon has already tried to kill the site, and he banned the owner from Twitter, of course).