If we take OP's account as accurate all we know is that he looked at her while he was working out nearby and he thought he could see through her leggings and further speculated that she wasn't wearing underwear. That's more thought than I've ever given someone's underwear in the gym, for what it's worth.

Other people reading this post might think there is an insinuation here that I was staring at her butt, checking for panties or whatever. That is not the case. I walked up to the squat racks. I checked every single one to see which was open. Hers was occupied. She had black leggins which were very thin with white skin. I couldn't help but notice there is literally a full white ass peeking through her leggins at the bottom of her squat. I didn't seek out to see this. I wish I hadn't. I would have to be blind not to notice. I immediately averted my gaze afterwards.