Dimwit scared the crap out of dad tonight. I was in my room and I heard a lot of LOUD angry bark/roaring. Dad had walked into "Zaphod's" garage when Z was crated and I wasn't present. I came out and Z quieted down, he rumbled at dad for a moment or two but was ok when I shushed him and led him over to dad. He let dad pet him and all was well, but he's very, very watchful around him. We haven't had a strange man come around yet, I'm not sure how he'll do with that. He's great with everyone when I'm around, but apparently significantly less friendly when he's alone.
Surprisingly dad wasn't disturbed by it, just admitted that Z was wary of him and was being a good dog for protecting his space.
In the future, I think I'm going to have dad throw food at him every time he goes into the garage and doesn't get barked at.