Some thoughts are
1. Belly band him when he is in some else's home.
2. Keep him on a leash so he can not sneak off.
3. As I do not believe dogs mark territory but do use pee to talk to others it more likely he is stressed and is peeing do to it smells of him and comforts him.
4. As Dogs do not generalize he does not know that he is not to pee in someone else's house so he just needs time and teaching from the start just like at home.
5. First thing to go under stress is the last thing and the hardest thing to remember.
In all those points I go with he not generalizing his skills and is under stress.
JL
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"The truth about an animal is far more beautiful than all the myths woven about it." Konrad Loranz |