Well, when we had out first tabby, we has 2 seiamese (sp?) which someone killed both with a gun in our front yard for no reason.

He acted depressed for a while, but my mother found the second tabby as a kitten in the park. he looked less than 6 weeks old. We took him home, and in a few months he was better. I doubt that could be it.
One guess is that one of the cats is having a urinary problem, and is peeing on thngs to tell us that he doesn't feel good.
Also, the first tabby was an indoor/outdoor cat before the seiamese (sp?) were killed, and we kept him indoors since. Now both tabbys keep trying to sneak outside when we open the door.