What could be wrong?

[CarbeDiem]

I'm cat sitting as I often do for PIL and their kitty is driving me mental.

He's a persian, roughly 12 years old and healthy.
He was, until around 1 year ago, an indoor cat but has been allowed to go out during the day due to PIL staying in our property which has a garden.
He has the choice of 3 clean litter boxes throughout the house which is 99% of the time used by him only. (our cat is an outy and only uses the litter box if desperate)
He is pissing on things and I'm pissed off with it. He's pee'd on dh slippers, fil work shoes, my bag and just on the floor - all of these things were in the same area each time (and literally a few footsteps from his bastard litter tray) and god knows where else.
Dh and I haven't been here for the last 8 or so months and the first thing I could smell on our return was cat piss :( The carpets and rugs are ruined. He does have 'previous' but I couldn't be sure and Mil poo poo's it and doesn't admit he does it.

In case it's relevant -
Pissy persian and our cat live together and have done for ages, they don't love each other but tolerate one another - no fighting, the odd hiss or swipe but that's it.
He has been castrated and he eats a mixture of Gourmet pouches, raw beef and dry food - albeit not much but he drinks quite a lot.

This morning I lost it - I haven't before because I hadn't caught him and it didn't feel fair that I tell him off when it 'could' have been my kitty. (I knew it wasn't) Carbecat was out all night with no access so when I hit the pissy floor this morning I grabbed persian, showed him the wee and shouted at him while pointing my finger which he doesn't like.

What can I/should I do?

[thecatneuterer]

The two main likely causes are either stress or a bladder infection. If it's only small amounts of pee then a bladder infection is the more likely. If it's big puddles then stress is the more likely.

Regardless of which it is punishment is always counterproductive. Cats simply do not understand the concept and to them you are just being randomly nasty, which will make them more stressed. So the 'shouting and pointing' could well make things worse.

[thecatneuterer]

Oh I forgot, he's male. So in that case a bladder infection isn't that likely.

[cozietoesie]

Have CarbeCat and him been living together in your 8 month absence? I don't know if CarbeCat is also a (neutered) male but I had this to a degree with Seniorboy when The Lodger showed no immediate signs of leaving. (He's a Siamese not a Persian and I'm afraid I don't know whether Persians are equally territorial.)

He always pee'd/sprayed by his box and not inside things so very easy to clean. He didn't do it often but it was starting to become more frequent and was clearly a protest: a real 'Oh Gawd, he's still here - so take THAT!' (He would do it in front of me at times.)

Pure stress at having another cat around. The Lodger was possibly the nicest cat you could ever encounter but Seniorboy loathed and despised him - as he does every other cat. The minute The Lodger left (and Seniorboy had checked every inch of the house to confirm his absence) it all stopped.

How long are your PIL staying with you?

[CarbeDiem]

Thanks for the replies.

The pee puddles aren't small, probably what I'd imagine a normal wee size.
I really don't wish to be nasty to him and usually - for example - if he's thinking about climbing up on the bench/table and I point my finger at him and shout NO! he 'understands' and won't do it Carbecat doesn't like it neither and I think it's more to do with my voice getting higher. So my reaction this morning was a) because I was angry and b) hoping he would get it.

Carbecat has lived with him in our absence, but he's lived with him for long periods for around 4 years now.
This is carbecats home, he normally was alone, no other kitty contact except his friends outside so I'd expect him to more territorial than persian.
Persian has previously lived with his 2 brothers before living with carbecat so shouldn't really have issues - should he?
Yes carbecat is neutered too.

Like I said before - they both don't particularly love each other but they don't hate one another. They'll sleep on the same sofa, eat together etc..
It's a strange situation we have.
PIL often travel around the UK for FIL work (locum Dr) and we go abroad for dh work a lot.
We have taken Carbecat with us many times but it became too much for him - the car travel, the new surroundings (tiny apartment) and the enforced captivity to an outside cat was just too much for him - he became very depressed :( The last time we brought him home to the uk and let him go free in the garden, I swear he actually smiled - we both said we wouldn't do it to him again. So PIL look after him for us.
We often have persian while pil go on holidays but they've been staying in our house all of this time due to fil taking work in our area - it made more sense that way. PIL didn't need to stay in accomodation in the hospital grounds, carbecat got to stay at home and the house wasn't left empty.

Oops! sorry for long reply :)