User blog comment:God of nothing/Shiba clan logic/@comment-6342763-20130428100445/@comment-5551986-20130428132911

From what we could gather from manga, anime, Isshin's unconscious appearance in front of Aizen with Ichigo. He has changed both physically and spiritually. He has aged alot more, and looks nothing at all like he did during his time with the Gotei. Rangiku may have recognized him when she was grieving over Gin, if not for his changes.

And his reiatsu seems to have changed. He had more before he lost his powers, so there could be a given then. And the fact from what we know about the Gotei. They believed Isshin died in battle.