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The dog is always exercised well, he gets several walks a day and spends a lot of the time running around outside (my bf works outside and takes him to work with him).
On top of that, the dog is taken for a good long walk late at night yet still has accidents sometimes so that obviously doesn't solve the problem.
We usually shut the door but obviously left one slightly open (not on purpose) so he got out.
We have another dog who will not go up the stairs at all, even if you were to stand at the top of the stairs and call her, she would still not go up the stairs, as she knows she is not allowed.
Reiterating my original questions, could it be that he is upset by the change in routine as he has never done this before?