I'd check the litter boxes to see what's in there... if there is anything softer than normal, it might be one of the boys. Also, check to make sure the normal number of "deposits" are in the box... a few short the normal could also point the finger at the boys.

I know once Nicki peed in the house and I was SURE it had to be Allen because Nicki NEVER pees in the house. Sure enough it was Nicki because she did it the next day in fron t of someone - we took her to the vet and got her all sorted out. When you have mutliple animals, its a guessing game as to who has done what. Good luck!