I always had a theory that Isshin was part of the zero squad at one point before leaving soul society. It made sense to me at the time since a lot of the captains didn't seem to recognize the name kurosaki when ichigo first made it to soul society.
I was actually just thinking about this. I wonder if Kurosaki isn't a fake name or something, because if it does turn out Ichigo is a Shiba, a former noble family, the captains should recognize any surname associated with it.