Could this be Ovarian Remnant Syndrome? (kittens, male, litter, house)

[CarnivalGal]

We adopted 2 cats from the shelter on the same day about 15 months ago, a male and female. The male was about 3 months old and the female was about 4 months old, so not litter mates. They get along very well. The male was part of a litter of kittens brought in as an owner surrender. Basically, someone's cat got pregnant and when they were old enough to be adopted, they brought the litter into the shelter. As a result, he has always been super comfortable with people, outgoing, friendly etc.

The female was brought in as a stray. I believe she was trapped, but we don't know for sure. As far as we know, she had no human contact before then. We were told she was "fractious and aggressive." But she's really not. She was just terrified at the shelter. She is really the sweetest kitty ever. She has never shown any aggression at all since we brought her home. She definitely had some anxiety issues, but they abated pretty quickly. She still gets nervous if there are strangers here and will pretty much just hide until they leave. Loud noises will scare her quite a bit too, so there is some lingering anxiety.

Here's the issue, she will go through periods where she is crazy affectionate - like she will jump on my lap and demand to be petted. Just sitting on my lap is not enough. She can be affectionate to a point, but demanding this level of attention is unlike her. She will also become kind of vocal. Not crazy, but she's generally not very vocal at all (unless she knows it's treat time), so even a little vocalization is really unlike her. That's all no big deal, but she will also urinate in the house. She uses the litter box 99% of the time, but will sometimes pee on a pile of clothes. It's always a soft surface, usually a pile of clothes.

At first, I chalked it up to underlying anxiety. But due to COVID, her life has been pretty stress-free. No strangers coming to the house, lots of attention because we're home all the time, etc. We have 2 litter boxes that are scooped daily, and totally cleaned weekly. Both cats use both litter boxes and have since we got them with no problem. We have those Feliway things plugged in all over the house. So, I started doing some research and am wondering if she has Ovarian Remnant Syndrome. She was spayed at the shelter before we adopted her, but maybe there is still some ovarian tissue was left and she's actually going into heat.

I have experienced a cat in heat before and holy hell, you could not mistake it. She is not nearly that bad. But I'm wondering if maybe there's like a "mild" heat if there is only some tissue left - like maybe less hormones involved. I tried "tracking" when these behaviors happen, and I can't seem to find any pattern. It also looks like cats don't really go into heat in the winter or colder months, like now. So I don't know. Does anyone have any experience with this or any ideas? I also want to add that they are both strictly indoor kitties and have been since the day we brought them home. They were at the vet about 2 months ago for shots / check up and were in perfect health. She's not showing any signs of illness, UTI, or anything like that.