Your dog is claiming this new territory, It is quite natural especially
if there is a other dog in the house. When you are there , you are the alpha figure and it is not necessary But in your absence
he is in charge and his marking is there to let dogs or humans know that he is making claim to this territory . the way to remedy
this is to catch him at it and tell him to stop . I all come down to training and if you keep at it ,It will end .