When a user gets de-synced in something like this, e.g. you connect the LDAP component to a new domain, they get put in Lost and Found.
So long as they have objects owned by that account ID, they'll stay in lost and found till someone does something about it.
If they don't own objects, they'll get purged eventually.
If one was so inclined ... you can see the ObjectGUID value in the DB under a particular usr table column.