There's also the possibility that there were no captives. Or rather, there were just enough captives taken that when these were allowed to escape, they would spread the false impression that their cellmates had also been captives - while in fact they were simply further Founder impersonators, ready to take over from previous impersonators who were in imminent danger of exposure.
Say, did Garak ever return from the prison camp? Or was Bashir just allowed to escape when the Bashir Impersonation Program came to a triumphant end - along with a false Garak who would subsequently e.g. pretend to suffer from shell-shock and fumble his code-breaking tasks ("Destiny")?