The question is asking why indoor cats spray urine, which is done, as you probably know, to establish territory. Because this is the purpose of spraying urine (normally onto vertical surfaces) it occurs when cats feel that their territory is threatened by an ‘invading’ cat. How might this happen to an indoor cat?
Some examples of situations which elicit indoor cat spraying
Normally the sort of situation that elicits marking territory by spraying urine is when an indoor cat looks out the window and sees a cat entering the backyard or coming near the home.
The indoor cat begins a defence strategy which is to spray a surface somewhere within their ‘home range’ perhaps nearer the boundaries of the owner’s home as they represent the boundaries of the cat’s home range. They may also run to the door and hiss or growl at the window. They watch the intruder with intensity.
When a new cat is introduced to a resident cat there may be issues of spraying because the resident cat feels threatened by the incoming cat while the incoming cat may also feel stressed because they’ve been removed from their home range and suddenly dropped into an entirely alien environment. They need reassurance and spraying urine reassures them.
In multi-cat households they might feel crowded because their home ranges have been severely compressed. This might elicit spraying from one or more of the cats.
Lastly, a change in an indoor cat’s routine which a cat perceives as a threat may also prompt spraying both to confirm to the unknown threat that this is their territory and also to reassure themselves by surrounding themselves with the scent of their urine.
If a female cat is unsterilised she might advertise that sex is available. This behaviour would increase during the mating season and during courtship.
On this topic, I have a visiting cat from a neighbour. My cat is not an indoor cat but he spends a lot of time indoors. When the visiting cat comes into my home through the cat flap, on occasions, she might spray urine near the kitchen sink on the countertop. She’s doing this because she feels a bit stressed in entering the home range of my cat. She feels a need to mark it to ‘claim’ territory or at least attempt to. I think she sprays near the kitchen sink because sometimes I put bleach down the plug hole and bleach seems to stimulate mental activity in domestic cat a bit like catnip. I think it can trigger this kind of behaviour. This behavior annoys me. I’ve considered stopping entry. However, my cat seems to enjoy her company.
The classic treatment for stopping spraying is to spay and neuter (sterilize) domestic cats particularly before spraying behaviour begins i.e. usually before six months of age. This should be effective in 80%-90% of cases. It was ineffective for my cat. I have seen him spray urine in my backyard perhaps because of this visiting cat.
In the examples above, of an intruding cat being watched by the indoor cat from a window, one way to avoid the indoor cats spraying under these circumstances to prevent her looking through the window or prevent the incoming cat from arriving in the backyard. That’s common sense but prevention is better than cure.
You will have to use your imagination as to how you keep a neighbour’s cat coming into your backyard. It can be difficult but people suggest motion activated water sprinklers as one possibility.
As to difficulties in multi-cat households, perhaps the best answer is to consider carefully whether you really do want to adopt another cat. A lot of aggression problems come out of multi-cat homes on my research. And if you can’t do that then you might consider a way of extending their overall territory by building a catio or catios or even a full-blown backyard enclosure to allow stressed cats to find more space and feel more secure.
Lastly, you’ve probably have heard of the commercially produced product Feliway which is a brand of synthetic pheromone, similar to cat cheek scent markers (marking territory through scent from glands in their cheeks). It is said to help relieve stress and calm cats. It is also said to keep cats away from an area where they have sprayed.
As a last resort you might consider a variety of behaviour modification drugs as recommended by your veterinarian. Personally, I would steer clear of these unless you’re really desperate.
Detecting sprayed urine and removing it
Sometimes you might be unsure if your cat has sprayed in the home but you sense they have because of a faint (or stronger!) odour. I think the best way to detect urine is to buy an ultraviolet torch (cheap on Amazon). If you use it at dusk when the interior of your home is dark it will pick up urine stains very positively. You remove urine stains with an enzyme cleaner which changes the urine chemically and nullifies it. On hard surfaces an ordinary cleaner will suffice.
SOME MORE ON SPRAYING: