Hi there,
I had this problem for a while and it's frustrating, to be sure. And once they find a spot, they go back again and again. So, have your vet check her for any problems. My Puddy has NEVER done that but when she first started getting a UTI, she peed on the carpet, hardwood floor, anywhere, but she wasn't sneaky about it. She did it right in front of me. That was a bright red flag! Once/if your vet rules out illness, I can tell you a few things I've learned:

1) Cats are peculiar about scents and odors. If there is a perfume, cleaning agent, air freshener, etc. that a cat doesn't like, chances are good that they'll pee where the offensive odor is. (That could explain why she was peeing on your ex's clothes. She prob. didn't like the girlfriend's cologne.) I had a friend that Boo, one of my males, didn't like, or so it seemed. He'd wait 'til she sat down, then come up behind her and grab a mouthful of hair and yank it as hard as he could! After the process of elimination, we figured out that it was her hairspray that was offensive to him. She stopped using it and he stopped doing it. And Boo is the most easygoing, cool cat you'll ever love.

2) If you've recently introduced another cat into your home, a cat who's already established in the home may pee to mark territory. (But I'm sure you already know that.)

3) Sometimes a cat will do a "spite pee". My male, Specs, used to do it when I had to work longer hours than I do now. Since I'm home more, he doesn't do it. When I would walk in the door, he'd make eye contact w/me, then spray right in front of me. I thought he was telling me that he was sick. No, he was telling me "Mom, I miss you."

Hopefully, these things will help you out but don't neglect going to the vet and having her checked. Then if she still does it, start going through the check list. Good luck.

Blessings,
M