12 month old border terrier

Question
QUESTION: this dog has been house trained since ten weeks old.recently on a morning,after he has been outside he has been sneaking upstairs and urinating in the bedrooms.sometimes it is only a few drops and he never does this at any other time.my back door is always open during the day so this is not neccessary.he has never been allowed upstairs and has never attempted to before.i also have a 5 yr old lab who has never done anything like this

ANSWER: What is your question?

Block it so he either cannot get upstairs or go into the bedrooms.

---------- FOLLOW-UP ----------

QUESTION: the queation is why would he start doing this now

Answer
He is probably doing this because he has entered adolescence and is feeling the need to mark his territory as he enters maturity. He marks his territory outside as he goes around the property, is clean in his areas of the house because that is what he has learned, but he was never taught that the upstairs was also off-limits for pottying because he was never taken up there. Newly discovered territories require making a statement.... "I am here, and all this is MINE!"

Either train him that upstairs is also a pee-free zone, or put up a gate at the bottom of the stairs.