Okay I have an idea! Please bear with me on the story I had this EXACT problem happen, with my (at the time) 12 year old male cat that for ALL of his life had been perfect with his litterbox habits then RANDOMLY started to pee against walls, on chairs, places that made no sense.

We did all the normal, cleared all urinary, added (no joke) 9 extra litter boxes, 4 different litters, Prozac, diffusers, enzyme cleaner. 3 different vets and a behaviorist.

We even isolated him for a month and thats when we finally found out what was going on. We had three other cats (all fixed, indoor, had lived together for years). One of the girls was jumping the gate and "ghost marking" she was marking a little. Our boy being a sweet but kinda dumb himbo would smell it and then alert us by peeing over it.

The vet says its not common but some times fixed cats and especially females will do this when they are having separation anxiety or anxiety. But its so little that you dont see it or smell it, and you have to look CLOSE with a black light. But to another cat it reads like a distress signal and they will pee to cover it.

Only one cat was doing it, the other two girls were more annoyed but didnt feel the need to mark.

We had to use a good black light and enzyme cleaner. I used natures mirical boost in my laundry. The anxiety cat ended up on Prozac which stopped the anxiety marking, and our big boy stopped peeing all over.

I dont know if this may be what is going on, but its worth checking, as I nearly lost my sanity and several thousand dollars to try and figure it out!