I think they get disoriented in a new place, and he just needs to be watched more closely and let out more frequently "just in case" he needs to go. It's so different when you leave them with someone else b/c they usually don't watch or know their every nuance regarding toileting.... I know my mom wouldn't think that much about it, b/c she's just not used to having a dog inside, so maybe that's the problem. She just needs to watch him more closely.
I know how you feel b/c when we took Max to grandma's the first time, I was telling them how good he was about going outside....Christmas morning, my stepdad stepped into a little "present" in the office
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