I hate to suggest going to the vets but, that really is what you should do. Your cat could possibly have dementia, possibly finds walking to the litter box if it's further away to be painful on Old joints and just finds it easier to go where it's convenient but most likely, is just following his nose to what to him smells like where the potty is. Even if you can't smell it and if you can't, you're blessed, definitely 100% no matter what you've cleaned the area with, your cat can smell it.
There's a million cat urine cleaning products out there but I'll tell you as someone who's used literally dozens of them, none of them work very well. (I too had an elderly cat who started weeing in the house, which then set off to other male cats in the house who never had a problem before, and well let's just say it was terrible)
I will tell you that if you can get hydrogen peroxide, that is one of the best things for cleaning cat urine. Now there is of course something to be cautioned about hydrogen peroxide and that is that it can bleach fabric / upholstery/carpeting. It doesn't always but occasionally it can. But for bathrooms you're probably safe.
You're going to want to clean and then clean and then clean some more. So, I would say at least two good cleans, if you want to start with your favorite floor cleaning product to get the floor to what you consider to be clean, and then if you have a steam mop or, if not, just very very hot water, just plain water, (of course being careful near the toilet because of the thermal shock with the cold ceramic and hot water). Then you're going to want to do at least one pass with hydrogen peroxide. Have many rags or paper towels, you don't want to be spreading any trace of urine around on your cloth.
Many many cloths, do an area, then throw the rag or paper towel away. Then use a fresh one, do another section. Throw that one away etc.
Hydrogen peroxide is, not my opinion, but proven to be an excellent cleaner of cat urine which is one of the most powerful domestic animal urines you are going to run across (again, not opinion).
Once the bathroom is clean, that might help, but it may also just be force of habit for your cat to revisit and soil the area.
Is your cat's litter area well lit? But not too well lit? They don't like it to be super bright but on the other hand if they're eyesight is starting to go, it might be difficult for them to walk in the dark to find their litter box at night, so something like a night light would be handy to help lead them to their litter area.
If you're using scented litter, try switching to an unscented litter. A lot of cats have an aversion to the fragrance that's added to litter.
If you've changed litter types recently, as in you used to use clay and now you use cedar or pressed paper pellets or some other type, some cats are extremely specific to litter texture and they don't like the change.
If the cat's litter box was far to walk to or downstairs, it's possible the cat might be experiencing arthritis and finds the Trek to be too far or too painful.
There are a number of factors, and the reason I hate to recommend that which is the number one thing you should do (which is go to the vets) is because they honestly don't usually have much of a solution, and it requires lots of testing of kidney function, blood test, and really, this doesn't really tell you very much.
I would say try the easy things first. Make sure the litter box is more accessible, and if need be, even though it may be unpleasant to you, consider putting a fresh new litter box in rooms that normally you wouldn't put a litter box in like possibly a bedroom or some other room. You have to treat your elderly cat who has this inappropriate urination problem as you would an elderly person who's got incontinence. They're not doing it on purpose.
I really hope that this helps. I know how incredibly frustrating this problem is.