I'm going through this same problem right now. Somebody peed on the baby bibs that were in a basket on the kitchen counter. Main suspect is Pouncer, who is susceptable to urinary infections.

Just watch all the girls closely and make notes (mentally or on a paper pad if neccesary) of who goes to which box, how long they were in there, how big the bicuits were and how often they visit the box. I NEVER see Pouncer in the litterbox (he's a very shy and private little boy) but I KNOW he's sick when I see him in the box - because he generally never leaves the box when he gets really sick.