So I was re-reading last weeks chapter (515) and as I got to the part regarding how Unohana was ordered not to leave the hospital and enter into the fight...is it possible that she was ordered not to go outside/fight, not because her primary mission was to care for the wounded, but because Yama didn't want the enemy to KNOW who she was?
Granted, some might say that if they knew where Aizen was held, they should know about her. But bare with me a minute.
It has been tossed around that Unohana could have been a silhouette in the picture with Yama when he was younger...what if she went under a different name then and Bach would recognize her if he saw her but doesn't know the Unohana is that individual under a new alias.
I think it could be an interesting idea to ponder since she has such a mysterious past. Could be anything, and like many, I think her past will be epic.