As a command module, Bit has the following ability: "Doors to safe zones are always unlocked. Security units with [Unlocked Doors] as their patrol will ignore these doors."
Some security units have the Cautious ability: "If this unit ends its activation adjacent to an unlocked door, the door is locked." When Bit is the command module, this ability does nothing against safe zone doors, as those doors are always unlocked.
When playing against Chrak as the captain, Chrak's burncycle action is: "Lock an unlocked door of your choice." You cannot choose to lock a safe zone door, you must choose a door that can actually be locked.