I think your refrering to "Another Report" where its says, and I quote "VIII - When the heartless are defeated, what becomes of the stolen hearts? Also, when members of Organization XIII and other Nobodies are defeated, do they return to their original form?
When heartless are defeated, essentially the owner's hearts are rejoined with their once extenguished body, whichever world it may be on. As for the whereabouts of hearts in KHII that turn up, this time they remarkably went to the organization (there is a foot note here that says "In Kingdom Hearts II after heartless were defeated, the many hearts were absorbed by the Kingdom Hearts of "people's hearts"). However, in the rare case that the body changed into a Nobody, when there is no container for the heart it resorts to a state of suspension.
In the case of a Nobody being defeated, it's a little more complicated. If the above mentioned hearts are liberated, they return to their original form. However, if the heart is still stolen by the heartless, the Nobody's body is swallowed by darkness. If somewhere in the world their hearts are taken back, perhaps they might be able to return to their original human form. "
