xbox360 Player is unable to leave the briefing room (door is locked and requires a key) after using speech skill to convince Orion Moreno to stay. One work around is to pickpocket the lock down code from Orion Moreno. Another work around is to kill all the remnants. They will not show up at Hoover Dam but Arcade Gannon will not be hostile.
xbox360 Player is unable to leave briefing room after killing Orion Moreno due to the key being removed from inventory upon quest completion. No other NPCs in the room have a key, and the only option is to reload a save. the Remnants will still help you during the final battle, but the quest will never be completed.