Something very similar to this happened to me in 2004 or 2005. I would have been in my early 20's, it was University days. I have told this story before perhaps on the old forum. I have no video footage, so you just have to believe me (or not) but I remember it still clear as day.
I had gone to a coffee shop I regularly went to back then to read a book for one of my courses. I was sitting on the back patio of the coffee shop on one of those lawn furniture chairs through which you could see the back of my jeans. I was wearing panties as I always did in those days, and on this particular day one of my all time favourite thongs I had commandeered from a past girlfriend. It was striped in various shades of pink and had a big pink butterfly at the crux of the thong with a couple of shiny sequins on it to make it particularly stand out. At this time I had never intentionally flashed a thong before, I didn't even know this was "a thing" as they say. There was a group of four girls sitting behind me who I didn't know per se, but who were regulars at the same coffee shop and I knew them by sight. It had occurred to me that they had become awfully quiet and giggly, although I honestly in my naivete at that time did not equate that to anything.
The "ringleader" of these four girls was a beautiful blonde German girl who as I was to learn had that confrontational/forthright German personality.
To my shock and horror, a few minutes later she approached my table standing directly in front of me and asked me, deadpan: "Why are you wearing women's underwear?"
This automatically elicited further giggling from her other three friends.
In my complete shock I came back with an idiotic denial while at the same time reaching behind me, my hand confirming that the thong was in full view, prompting even more giggling. And I was sitting in such a way that I couldn't easily adjust it out of sight, so I was stuck with it hanging right out there.
Still completely deadpan, she said "Yes, you are. It is a pink thong with a butterfly on the back."
Blushing and stammering I said "oh yeah...um, well I just find this more comfortable."
She smiled and said "OK, that's cool." And returned to her table. There was a little more giggling but it died down. I did not want to leave right away and act totally scared, so in defiance I sat there for another ten excruciating minutes or so knowing it continued to be in full view, before I left thinking I'd stood my ground long enough.
I actually continued to see that girl and her friends around on subsequent occasions. Never again was anything said although this girl whose name I had learned was Agata, would sometimes wave to me in all sincere kindness.
About five years later I saw that same girl in a pub at night, again out on the balcony, where as luck had it we were the only two people out there (she had come out for a cigarette.) She came right up to me and asked how I was. After a little bit of small talk she came right out and asked me if I still wear women's panties.
Having had a number of years to reflect on and relive that first incident, and knowing nobody else was around, I plucked up the courage to take advantage of this conversation as I had fantasized about ever since the first instance.
I looked her dead in the eyes and with no hesitation answered honestly: "Yes, I do. I'm wearing a pair of lace panties right now."
She smiled and said "that's awesome."
And such was my interaction. If you wear panties enough, it will happen. I reasoned that your underwear is always likely to show at times sooner or later, no matter what you're wearing, and if you wear panties 24/7 it's only natural that they will show sometimes whether or not you're aware of it or whether or not anyone ever says anything.
I still think on that day often.
IF ONLY I HAD A RECORDING! I think I'd still watch it every day.
My fantasy would be to have a purely accidental whale tail flash in front of a girl who knows me, she would confront me asking "why are you wearing women's panties?" I would blush and deny, but she would state "yes you are, it's a pink lace thong with a pink bow!" and give me a wedgie, then laugh her ass off about it. She would ask all sorts of embarrassing questions like "do you prefer thongs?" "is it a fetish thing?" "you better not have worn any of MY panties! Oh wait, I don't think I wear panties that girly." She would agree to keep it a secret but still joke and tease me about it regularly, when alone she would loudly ask "so, what color are the panties you're wearing today?" with a big grin. Eventually she would notice that I'm always wearing a different pair and say she's impressed that I have more panties than her, then ask to see my collection. When looking at my panty drawer she would have another laugh out loud moment at my expense, and suggest that we go lingerie shopping together.