I'm sure we will get recomendations and these must be acted on universally by the industry to ensure that nobody can be left alone in a locked flight deck door.
2 sensible ways :
Key or RFID type Fob in the cockpit that goes with any crew memeber when the leave the flight deck, that will allow access no matter what security setting the door has been placed on.
Upgraded door system to allow a per flight specific random door code to be generated that would override any security settings.