What if the pilot was having a medical issue and sent out the other pilot for help, but still locked the door until the other pilot returned. Maybe he died before he could unlock the door to the let the other pilot back in. Small chance of that but still possible?
This plane was already at cruising altitude which more than likely autopilot was set, which maintains altitude unless someone changes it.
A pilot doesn’t leave the controls to get medical assistance. He/she would call one of the flight attendants as they are trained to handle medical emergencies.
Why hear a knock and no voices?? And then bashi but no voices...like Hey Joe...you okay...open the door...