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.