My 3yr westie has started peeing in my living room, he isn't allowed in the living room on his own and we never see him doing it! Which isn't helpful.
But just now, after being looked out the living while we had dinner, once he was allowed back in, he peed! What is wrong? How can I stop it?

How are you cleaning up the "old" spot(s)?

The answer is to watch him more closely - and I know that takes extreme, extreme vigilance and a lot of time and energy.

Sounds like it could be a marking issue. Seeing as he is not allowed in that area, once he gets his chance, he marks "his" territory. I also would like to know how you are cleaning the areas?