Lori, I'm so sorry you're going though this.Unfortunatly, I don't know much about this, but like the others say, try a video camera first, or/and the Fluorescite, so you can find out who is peeing.
I believe it's very commom when you have many cats together that some of them want to mark their territory. When you have found out who it is, you should try Clomicalm or Prozac, it seems to have done wonders in some cases.
Best of luck solving this!
(((hugs)))
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