You'd known for a while the dog was scent marking, but you didn't realize things had got to this stage. It seems you had gone nose-blind to the background smell of pee. It was only when the house was shut up and the stale air built up, that you were awoken to the awful truth... your house smells.
What to do about it?
However, certain factors, such as being an intact male dog, will ramp up the provocation to scent mark. Indeed, reducing stress as a whole, along with desexing the dog are both constructive ways to reduce the problem.
Any crucial eliminate is taking down those 'posters' with effective cleaning, so that the dog isn't tempted to go back and reinforce the message.