The Key was one of the Items during Early Access.
This item was only obtainable after all the other items had been picked up by the players. When picked up, it would show a way-point to the door/exit it would unlock and this was shown to all player.
The Key was removed near the end of Early Access and is not currently used in the current patch of Deceit. If the key was not used to unlock the door/exit before the blackout ended, players would die to gas. Due to excessive trolling and new players being unaware of what to do with the key, it was removed and replaced with an automatic system which opens 3 doors/exits after the blackouts.