Flight plans still have to be filed, right? And are any of those details public, as in would a flight tracker show the destination of a plane in flight?
If so, time for some traditional intelligence work: someone at the airport with a telephoto lens to document just who disembarked from the plane.
The flight plan might include the registration number of a plane but if the ownership database is anonymized then it would require a fleet of "aviation enthusiasts" to determine on their own who owns which planes and then cross-reference filed flight plans with such a private database in order to dox the movements of celebrities. Would probably be an interesting business opportunity if it wasn't a giant "sue me!" target for people who have lots of money and are willing to drop $100k in lawfare on someone to make them go away.
If so, time for some traditional intelligence work: someone at the airport with a telephoto lens to document just who disembarked from the plane.