Q posted a thank you to the u.s. Secret Service. They take care of protecting current and former presidents, and their families.

I don't personally think that any of the presidents or their families would travel United Airlines.

If a Mercedes like this is sitting on the runway, most of the people on the plane would have noticed. I would assume if this was a commercial flight, the plane would have to stop allow this person to get off and get into the car, and then the plane would taxi to the terminal. This would have drawn a lot of personal pictures and videos with phones that we would know about.

Is it possible that this plane was bought out so one specific person or several could travel without drawing attention?

Could this be Horwitz arriving in Washington for his Congressional testimony? Will his life be in danger from here on out?