I think it really depends on the dog. My boy Jubel marks outside on walks but has never marked in my house or anyone elses. We went through five different short term fosters that we watched while their long term foster was on vacation, all females, all had at least one accident in the house during their stays. No marking really just accidents. Now I have a long term foster male, been with us just over 5 weeks now, who will mark in the house. Not accidents, marking. Through all of this Jubel hasn't marked or had an accident in the house. I've definitely heard of other dogs starting to mark in the house in response to a new dog in the home though. So yeah... depends on the dog.