I'd done a number of PTY same plane turns, just staying at the gate with no issues. Then as it turns out I sat next to an air marshall, who queried me on my return - I thought he was also doing a mileage run, but turns out he was clueless, even after I explained it.

Next time I was met by 2 customs men at the gate who interrogated me for 10 min while going thru everything in my bag [including reading my notebook] - all without my permission - clearly never having heard of mileage runs. I referred them to the gate agents who were around to verify, but they wouldn't talk to them.

Finally they let me board [last on], but the plane took additional delays both ways due to "flow control" - something I never heard of for PTY, nor did any other plane experience that the same day. Coincidence? Who knows. On return, I sailed thru customs, so sounds like they were just doing something for their own job security [if air marshals don't actually prevent any attacks - they have to justify their job of sitting there somehow].

Now I'm taking with me a printout explanation of what mileage runs are so they can "document" they're presumed reports. Most customs people have heard of MR's, and just laugh, but some with a badge are just ignorant, intrusive, and arrogant.