It’s a fine line against stalking, so maybe do it with a few hundred of your friends.

Also NAL, but that's an interesting idea. If I constantly follow Zuck around, might be stalking. But if I spot him Mon at 3pm and never see him again in my life, and someone else Mon at 4pm, and someone at 5pm... I wonder if that's a legal loophole. Tho, might be hard to spot him at all.

Even privatebjets emit a transponder signal

There was a big controvery over Elon's transponder. Also for Taylor Swift and other very high profile celebs. I lost track of where it ended up. From here,

The most prominent was @ElonJet, which tracked Elon Musk’s air travel and amassed more than half a million followers in 2022. The account’s creator, then 19-year-old Jack Sweeney, became embroiled in a public feud with Musk after rejecting a cash offer from the Tesla CEO to delete the account. Sweeney was eventually banned from Twitter after Musk acquired the platform in October 2022.

Ofc just being banned from Twitter doesn't effect the data, which is still available on public web sites.

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).