I'm fighting the same battle you are. It's so frustrating. In my case, I don't know who it is. I can guess that it's one or two, and I think I know who would be most likely. But that doesn't solve the problem.

I have one guy who will definitely pee if he has to stay inside. He also chewed the base of his tail until it bled. So now I let him go out, but try to keep him inside only at night. That isn't working very well. I suspect there is at least one more cat involved.

I wish I had a solid solution for you. I just keep cleaning up the puddles and cursing under my breath. Simple Solution better not go out of business or I'm in trouble.