No, He's Not in the Army

Today I was having lunch with friends at a local restaurant and an older man approached me and gestured toward Isaac and asked "Is he a sergeant?" I had no idea what he meant and I guess I looked confused, because he pointed at Isaac (like maybe I hadn't understood who he was asking about) and repeated the question.

I just said "He's a service dog." Because I didn't understand what he meant and didn't feel like trying to get him to explain it.  Maybe he thought Isaac was a military dog or a police dog or something.

When I said he was a service dog, the man nodded and then told me how beautiful he was. I told Isaac he could say hi, which delighted both Isaac and the man.  The man told me repeatedly how good looking Isaac was and that he looked very healthy.  He was actually the second person to comment on how healthy Isaac looks today.

But I still have no idea what he meant by sergeant.