I believe it is highly likely that you have more than one pee'er. I also believe that Tiki felt forced to use to use a container to urinate in since you had locked him out of his litter area, even though he could have walked around to find another box.

I can't count the number of times I have been totally sure that I've known who the pee'er or sprayer is only to find out that it is one or two cats that were barely on my "might be" list. Only a few days ago, I found out that cat spraying problems I've been having for weeks now were not caused by the two cats that I had decided it had to be - both because they had backed up to furniture and because I'd found it had cat pee on it as I rushed over. They were actually reacting to what Thumper had already done and I hadn't seen, as I understood after I'd isolated them. I was stunned when I saw Thumper in the act since he's highly sociable with other cats and didn't appear to be territorial.

Have you tried isolating individual cats for a week or more?

I should add that I find cat pee to be the hardest thing to deal with in a multiple-cat household, Lisa (K&L) have commiserated with each other on the issue. While my disabled kitten was learning to use special litter areas recently, I found cleaning up the pee from carpets far harder to cope with than the poop.