HMMM...
Monty has been peeing outside the box ever since he was very young. First he wanted to pee on the bath mat, so ever since then we've not left it down on the floor unless we're actively using it after a shower. Problem (temporarily) solved.
Then about a month later, he started peeing on our bed. He didn't ever spray, and he would only pee there if he already needed to pee. VERY inconvenient as we had to keep dry cleaning our big, heavy and expensive quilt. We also tried the enzyme cleaner to try to properly eliminate the residual smell. Eventually we had to close the bedroom at all times. Some nights, he would scrabble at our door and then he'd give up and we'd hear him in the litter box. It seemed his first choice was our bed but if he couldn't access it, he'd go in the box.
After about 3 months of having the door constantly shut, we tried again. He was perfect for a YEAR. But then he started peeing on the bed again. We don't know why. Nothing had changed.
We had him checked thoroughly at the vet. Nothing physically wrong. We have had Feliway going ever since (diffuser).
So we shut the door again and it's been shut ever since (3 years ago I think this is).
All was well until we noticed staining on the carpet in the dining area. Seems he had been peeing there. Not every time, but only when we weren't around to supervise him. Our routine is he has his breakfast at 6am and then he is shut in until he goes to the toilet. Once he does, he is let out with praise and a treat. Same thing at dinner. But sometimes, he will pee several hours early while we're asleep. That's always when he does it, overnight when we're not there. If we're around, he'll go in the box.
So he started only peeing in one corner of the dining room. We sprayed with stuff cats don't like the smell of, we sprayed with Feliway spray, we sprayed with enzyme stuff to get rid of the smell, we had the carpet professionally steamed and shampooed, we put sticky paper all around to deter him from going in that area. We even put ORANGE PEEL there as they're supposed to dislike the smell. Nothing worked. He just moved over to the next area along from the corner. For a while, he seemed to stop and we were pleased. But then we realised he'd been peeing in the study. UGH. So now THAT door had to be shut all the time also. Then he started peeing outside our bedroom door. That didn't last very long though.
Now he has moved over to a whole new area in the living room itself. This began after he peed in his own bed twice. The second time was after we washed it with enzyme cleaner and took it away for a week before putting it back. A few hours later, it was peed in. So now there are no beds either. Not that it's cold enough for them to want them right now. But since we moved the beds, he has now started peeing in the area where the beds WERE.
We suspect he just likes soft furnishings for toileting. I've read about that. We've tried putting soft things in the boxes, but Dweezil came along and scrabbled at them until they were out of the boxes. LOL. We cannot separate their boxes or the cats. We live in a modest 2 bedroom apartment where they cannot access our bedroom or the study due to the pee problems, so their areas are the living / dining room open space and the kitchen and bathroom. The kitchen is too narrow to fit their cat food or water in it. So the large bathroom has one area for their eating stuff and another area for their boxes. We also suspect this is not adequate for him as Dweezil pees in his box and poops in Monty's. But Monty pees and poops in his own box, so he probably feels like he doesn't have enough boxes. We don't want to have a box in the middle of the living room on the carpet. Also because Dweezil will use it too and he sometimes pees right over the side of the box (doesn't drop his butt down at all).
We tried having THREE boxes in the bathroom (which basically took up almost all the room and my partner couldn't brush her teeth without stepping in litter at times) but he still didn't want to use it. Back then, he was a bit worse in his resolve NOT to pee in the boxes. We would see him sniffing around his "areas" of carpet, so put him in the box. He would look supremely awkward and unhappy and jump out and run off. Then we'd put him back in, stroking and reassuring him. He would again jump out. And sometimes he would drop his butt and actually PRETEND to pee, but he wasn't really. He had no UTI issues at all, he was just sitting there waiting for it to seem long enough for us to think he'd peed. LOL. Then we'd have to go to bed and later we'd find he'd peed on the carpet. Luckily, he doesn't seem to do that anymore.
Even when he DID consistently use the boxes, he seemed to hate it, no matter what box, what litter or where the box was. He would step in, feet trying to rest on the edges of the box, do his thing and then literally SPRING out of it, litter flying everywhere, and run off. He never scrabbled or scraped or loitered in there like Dweezil does. We think because he's a rescue kitty and when we got him, he was kept in a cage and often slept in his litter box, he doesn't have that degree of separation. Maybe. Though lots of cats are rescue cats and don't toilet inappropriately. We also notice a few times when he's been very scared and has hidden, we've found him actually hiding IN his litter box. Like laying in there trying to hide. He's been physically fine, just scared.
Monty also poops on the carpet in the same areas as he pees. But it's ALWAYS just one or the other. He will often alternate weekly. One week he will pee a few nights (overnight as always) a week but always poop reliably in the box (even overnight), then the next week it switches to the opposite.
We are just hoping when we FINALLY move house (we've been trying to sell, find a new place and move for 3 years), it will help as we'll have more space for them both, more tiled areas for more litter boxes, and no existing pee stains and smells.
Talk about a vent!!!! LOL
I hear you. My civvie Monty also poos AND pees on the floor quite regularly and has done so for several years, but not consistently. 
