I heard on a tv talk show that in the United States, a cockpit must have at least two persons in it at all times.
For instance, if a pilot leaves, a stewardess must take his place so that there are always two persons in the cockpit at all times.
But the tv person wasn't sure what the law in Europe was concerning two persons in the cockpit at all times.
Not a law. Just procedure. Someone has to visually check the door before opening for the returning pilot.