Ironic you should mention this; a couple of really creepy things along these lines happened to me at my last gym.
First: I was working out & this guy kept staring at me. Eventually he comes over & says, "Does your husband or boyfriend work at Unitrode?"
In retrospect, I should have said, "They both do," just for a goof.
But anyway, I said, "Yes, why?"
He then went on to explain that he worked the cleaning crew there & that he recognized me from the pictures in my (now ex-) husband's cubicle. That totally creeped me out.
The second time was when a worker/trainer used my personal info in their database to call me up & invite me to a movie. My then-husband didn't appreciate that, either. Way uncool.
If you're asking because you are interested in someone at your gym and want to know a non-creepy way to approach her, I would say just offer her your card on the way out - when you're all pumped-up but
showered, smile as prettily as you can, and don't look back while you're walking out. That'd be the least creepy and most intriguing gym-bunny approach, as far as I'm concerned.