housetrained dog now peeing in the house
My 6 year old lab/chow mix, Sammy, has recently started peeing in a couple of places inside. He was housetrained when we adopted him at 1 year. Very good dog, intelligent and friendly. A few accidents over the years, but really very good at waiting until we let him out. About a 6 weeks ago we noticed a couple of areas where he had gone in the house. Of course this had to be on the corner of our nicest pieces of furniture! Now we are desparate. We have tried boudary spray to keep him away from these areas, and recently resorted to locking him in the garage (with food and water of course) while we are at work. This was never necessary before.
We think we know what started the problem. Our daughter brought her do over to our house (another male, but much younger). Since then, Sammy must be trying to mark the house as his territory. The other dog has not been back over for the past 3 or 4 weeks, but the problem continues. He must be doing it in the middle of the night when we're sleeping because he never does it when we are around the house with him.
My question for all you experts: What can we do? Can we retrain him? I
think he knows better, because as soon as we notice the spot and start to clean it, he gets sad and goes to lay in the corner. Any ideas? I hate to think we have to continue locking him up during the day or putting him in a crate day and night, but it may come to that unless we find another solution.