Are there any dogs at the other houses? He could be trying to mark his territory as well. It happens to us EVERYtime at petsmart, even if they go before we leave or outside at Petsmart, 9/10 times they will have to do SOMETHING once we get inside
I've also heard that everytime a dog is in a new envirnoment, like a new house, they have to get trained all over again for that place.