Who's on deck?
I guess you heard the JAL case a few years back?
The SOP was for the Captain to go to First class to have his meal. FO got a call from ATC to climb. Like a good JAL FO he replied "STBY", got out of his seat and went downstairs to ask the Captains peermission to accept the new flight level.
You guessed it. When he returned to the cockpit the door was closed and locked!
After much frantic chopping with the upper deck fire axe the Captain and FO, in full view of the upstairs passengers, regained entry to the cockpit and control of the aircraft.
SOP is now to ..............
Wait for it..................
Take the key with you when you go to ask the Captain.