What an amazing story! So..I've always been a HUGE lover of accidental tickling incidents like that...doctor's exams, pedicures, massages, pat-downs. So this kind of story is my absolute favorite.
I once witnessed an ex-gf (Darcy, who I've shared plenty of other stories about here), get patted down at a concert. She was squirming, wincing, biting her lip as the woman touched all the spots that I knew were insanely ticklish on her as I watched in utter blissful delight. She was on the verge of laughing just as the woman stopped.
I also happened to witness a pat-down at a different concert where I could see the girl squirming and then the second it ended she burst out laughing, like she'd been holding it in the whole time, and just started running away, like she'd been dying to just break free.
Pat-downs tickle me a lot, too. It's kind of amazing to me...that this is something that actually happens all the time, that people have to go through regularly at the airport or for events...where a stranger publicly touches, strokes, feels, slides their hands across all of your most ticklish areas. It's super awkward, and pretty hot at the same time.