Thanks for the reply. I have caught him a few times, and immediately taken him out, but I don't think it's because he has to go that much--he normally will bark or go to the door when he has to go out. A few times, he has been in the bedroom alone and done it too--we keep saying we aren't going to let him in there alone, but he falls asleep on the bed and looks so peaceful...

I thought it was a territorial issue also, but I don't know how to prevent it. We got Abbey about 5 weeks ago, but he did it before we had her. He is neutered, but he does still like to mark outside--I suppose that's what it is.