We had the same situation at our place however never actually caught the culprit, we had our suspisions but never caught him in the act which I think made it worse. On 3 occassions we came home to find the foul stench of cat spray in our kitchen and hallway and our 2 cat bowls licked clean (which is a very rare occorrence), one time it happened when we were actually in!!! The cheek of it!!

If it's any help, we did get a magnetic catflap and magnetic collars for our little fellas which solved the issue completely, I had been told tom cats are pretty strong and will get in but so far which ever one it was, hasn't.

Also, we hunted down the best way to get rid of the smell and found that using a mixture of biological washing liquid/powder mixed with hot water did the best job. It breaks down the "MINGING!!!!!" fatty deposits in their spray or something like that. The hardest thing though is to actually find where the bloomin thing sprayed in the first place as the smell takes over the whole house. I am invisaging people on their hands and knees sniffing the skirtings, cupboards and table legs as I write this!!!

Good luck with it, we feel your pain.