Labrador Peeing On Bed?

I have a 1 year old black Labrador. He is a very intelligent dog and since we brought him we never needed to train him. Earlier he used the pads or sheets we put and then began going outside whenever he had to pee or poop. Recently a friend left a female Golden Retriever at our place for few days and since then our dog has started peeing at a particular spot on our bed. He would go there when no one is looking and pee on the same spot everyday. Can somebody tell why is he doing this and how can I help him?

By Amber