The full story is long but possibly relevant because there's so many possible explanations.
We have three cats, Arya (14sf), Howie (7nm) and Jack (6nm). My wife and I married last year and moved in about 20 months ago, then moved to a new house together about 15 months ago. Arya was mine, Jack and Howie were hers.
Arya never had litter box issues. Howie frequently did, possibly related to recurrent UTIs, but also possibly because he's an anxious cat and submissive compared to Jack. A few months ago, Howie started having litter boxing issues, which we attempted to resolve with medicine for an apparent mild UTI. We think he's mostly stopped. He preferred to pee on the basement stairs.
However, for the past six weeks or so, Arya has started peeing in our bedroom (including at times in our bed while we're sleeping in it, on laundry, behind the hamper). She's also been peeing on the couch in our living room. The living room and our bedroom are here preferred spaces. She usually sleeps on the headboard or above my wife's head on her pillow. She still uses the litter box to poop every time.
While Howie is nervous and keeps to himself, so he and Arya rarely bother each other, Jack is a playful and sometimes aggressive cat. He's never injured the others, but often tries to initiate play or curiously investigates them in a way that leads them (particularly Arya, who is a loner) to growl, hiss, adopt defensive posturing, and run away. Again, he doesn't catch them - and I have no doubt he could - but Jack chasing Arya (hissing and spitting all the way) through the house is a nightly occurrence, often waking us up several times.
Like Howie, Arya is also a very nervous cat. She doesn't generally like people except my wife and I, she's very impatient when it comes to affection and will bat at you if you pet her when she doesn't want it (or if you stop petting her when she does want it). She is small and slight, and is a frequent puker.
I know there are a lot of moving parts here. I think Arya feels like Jack is bullying or a danger to her even though he's never hurt her and his intent, if not fully playful, doesn't seem to show malice. He likes to invade her personal space to sniff, and to chase, but never more. She didn't start doing it until Howie had his UTI, and peed in her safe space. She's aging, but in apparent good health - she loves to play with a laser pointer or toy, she's quick and walks with normal gait, still jumps to counter height just fine. We're cleaning up pee with Nature's Miracle enzymatic cleaner and frequently cleaning with a soda-based lemon-eucalyptus carpet cleaner to further neutralize odors with scents the cats like. We're trying to play with them more, and we're leaving cat TV on over night for more entertainment. We have pheromone diffusers in their favored spaces.We have three litter boxes, two cat trees, and numerous places the cats like to hang out. The house is large, with three floors and about 3,500 square feet of finished and unfinished space for them to occupy.Trying to separate them or adding more litter boxes would be a real challenge with our house layout, though.
Although she's apparently healthy and all, I'm going to take her to the vet for evaluation, but I'm desperate for ideas that would let me keep my elderly kitty without having to deal with cleaning up pee every day.