I have a 10 year old Shih Tzu. My sister watched my dog for 4 days and ever since he came back he has been peeing in the house only at night. He has done this for over half a month. I put his water up and take him out more at night but he continues to do this. I thought it was because he is getting old but I went back to visit my sister for a week and he did not do this once! He doesn't even get off the bed at night at her house. Why is he doing this? Why is he able to hold it there and not at home?