my lab has started messing in the house, all times of the day. I regularly let him out, leave the doors wide open, tell him to go, then I turn around and he's been in the house even minutes later. I've always had dogs and the only time that they have ever messed is right at the end of their lives, but he seems happy and well apart from the messing. It's been the past few weeks, I just don't know what to do. Anyone have any ideas?