Dog poops only in other homes.

[Boeriemore]

My dog never messes in our house but the moment I take him to my mother's home or my sister's home he eventually leaves a pile on the floor.
My sister has a dog but my mother has no pets so "marking out his territory" seems unlikely.
He has access to their outdoors at any time but chooses their lounge floor.
I find this embarrassing and initially insisted it could not have been him.
He is friendly and not a nervous dog.
Another thing. All day be has access to the outdoors but on the occasions I take him for a walk in the evenings he poops during the walk even though he has had nothing to eat all day.
Any suggestions to resolve this problem would be appreciated.