We have a cat that is 13 years old. He likes peeing in the house off and on, usually on blankets or in the dog cage. He does not do this all the time. We do have 4 cats in the house but 5 litter pans ...
My cat was diabetic, had to put him to sleep at 15. It appeared he drank normally but he did urinate on the floor occasionally. Some days were good for him and some days were not.
His blood work showed the disease, I am sure your cat's blood work showed it negative?
Hope your cat does better, it is hard to lose a pet you love.