(NSFW) Pants not opaque ... Would you say something? What would you say?

I was walking toward a subway station and saw ahead a young woman walking a dog. She is maybe in her late 20’s, maybe not athletic but fit, and wearing black leggings and a white t-shirt for the occasion.
The dog stopped and made a poop. The woman responsibly spent a few moments to pick up the poop with a plastic bag, bending forward and down at the hips to do that.

When she did her forward bend, the opaque, black tights she was wearing went suddenly almost completely sheer, showing legs, butt, thong (white with navy blue trim and strings) in near unobstructed view.

That was fun, but I wasn’t sure she intended to share that much.
I’m a grayed, middle-aged, married guy (wearing a ring) and couldn’t come up with a way to constructively mention the phenomenon of the transparent pants – in case she was not aware of it – while causing her a minimum of upset.

Would you have said something to her? What would you say? Should I have said something? How to avoid seeming creepy about it ?

Were those some of the infamous Lulleman yoga pants that are got recalled ?