takata, if they left the cockpit door unlocked while the captain took his rest, I believe that would be a security violation.
I assume Air France policy and procedures require the cockpit door to be locked, and that once locked, the door can only be unlocked from the inside. If that is true, and only the PF and PNF are in the cockpit, then the PNF would need to leave his seat to unlock the door and let the captain return, and the PNF would then again take his seat, fasten his belt, and see what had happened in the __ seconds it took him to let the captain in.