We have three and one was a late comer to the party.  So occasionally there is literally a pissing contest.  It helps a lot to be sure to clean all the old spots and, sure enough, I found some with the UV light. 
 
To anybody needing a UV light for this purpose, I can heartily recommend this light.  Pee spots show right up.  I had a cheap, pencil-type UV light and you had to be right on top of the stain to find it.  This one makes it really easy.