I have no inside knowledge but I would be surprised if the government does not have automated tools designed to move any and all emails meeting certain criteria to classified domain special folders. If so they could run rules and move all of Hillary’s emails on a certain topic like Benghzai to wherever. Backup data too. Surely the technology exists to do it. If something like that worked correctly no one would ever find those emails
Can be run as a bat file probably.
Or special rules in archive tab of whatever program they have.