Marty is two years old and was neutered when he was 5 mo. old. He is completely housebroken. Recently while visiting a friend who has a dog, I saw him mark in my friend's house! Just a few drops on a trash can. I was mortified! I thought it was an isolated incident, but he did it again at another friend's house who has a dog. He has never done it in homes that do not have dogs. Is there any help for this? Has anyone else had this issue?