Male Dog Marking in the House

My two and a half year old Westie (neutered at 9 months) has just recently started to make his "mark" inside my dog-sitters' house. He has stayed there many times before (once or twice a month for a couple days while I travel on business). They have a three year old, spayed Westie. "Marking" has never been an issue in the past, until the last three visits. He doesn't "mark" if I'm there, or if I leave him there for a couple hours. But, when I'm out of town, he has started a morning and evening ritual of hiking his leg indoors! He does not "mark" at home.

Any suggestions on how to stop this bad habit?

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 .