Also, the TSA (or other country equivalent) is usually at the boundary between public areas and the boarding/departure area, each plane has airline employees making sure that each passenger that boards the plane has a valid ticket for the flight.

The TSA has little to do with what plane you actually board.

it could and did happen. I remember reading about 6 year old kid getting lost at the airport gate (i think it was in germany) only to be discovered when they finally looked for their parents in italy. just went away while parents were preoccupied with something else and looked like someone else's kid while boarding the plane. as a parent travelling with small kid I can totally see that happening anywhere - kids can just walk anywhere with no questions asked as long as they are next to an adult who looks like they could be their parent.

aw man, I know people steal buttholes but fingering them seems a bit awful