They put the plane on auto-pilot.
The auto pilot wasn’t on the radar
That’s very dangerous while both are dreaming. Someone inside the cockpit must be alert enough to knock it out of autopilot and take instant control should some unexpected shit show up. If the two pilots are asleep then no one will get the warning.
You can imagine going to the cockpit as a passanger only to find two sleeping pilots. The panic.
I don’t think you can just walk in from the passenger side, they have to open for you. They take serious precations now coz of terrorist nutcases.