It does sound like a infection to me. Smokey pee where ever he feels like when he is developing crystals (stones) in his bladder. Fortunately for me I know his "moods" when he is not feeling well.

As for figuring out who it is....well that's the hardest part. My advise is to watch very carefully around the house, their behavior, their litter box habits, etc. you will probally catch whoever it is in the act.

Hope this resolves soon (((hugs))) I know exactly what you are going through and it can be very frustrating.