It doesn't sound like something was showing which shouldn't have been showing. Perhaps I'm not picturing it correctly. Could you send a photo...
That's a joke (and an example of creepiness).
I suppose the person could have thought it was a wardrobe malfunction, or, as I mentioned, perhaps it actually was a wardrobe malfunction, and I'm not understanding the predicament. The fact that they mentioned it seems to suggest they were being gentlemanly. Ungentlemanly would have been wandering around behind you enjoying the view and saying nothing.
Of course, it probably depends how they did it. If it were a simple comment and then on with his business than no big deal, if he was drooling and watched you correct the malfunction than that would obviously lean towards creepy.
That scenario has only come up once in my time. While walking on campus one day I saw a girl in front of me who had lifted her skirt up with her bag so her bum was hanging out a bit. As I pondered whether to say something, she swished her bag and corrected it, so I was off the hook. It's a tricky situation. Do you say something and run the risk of her being a little uncomfortable or not say something and risk her finding it out on her own and feeling terrible anyway thinking about how many people saw her butt?
My conclusion was that it would probably be better to say something but to do so in the least creepiest way possible and to not linger on the point at all.
At the end of the day sometimes you just have to go with your gut. Our guts can be pretty smart. Did it feel creepy or feel okay? Whichever it felt like is probably right.