@ErwinS - I have no cause to doubt what you say and think that it is said with the best intentions. But would like to add that the JTSB will most likely stretch a discussion like this into an independent Incident Report. It is not for the airline or maintenance organization, nor the manufacturer, to have the final say in what happened and why.
What surprises me and a number of others in this thread is, that on the photos that we have there is hardly any visible damage and the are no (bend or sheared) bolts or other tears or fragments. So what was the detachment sequence? Did the flight crew register it?
It put me on the track of thinking about what would be the difference in damage between hinged-panels (with failing latches) and non-hinged bolted-panels. In this case it looks like the panel popped or blew straight outward and did not hit the fuselage or any surfaces. Sounds special. And if it happened like that you could wonder if a pilot would be able to notice anything at all.
In another case of an 777, a hinged ADU-door of about 70lbs, detaching at 6,000ft, the pilots only felt a light shudder. And only investigated when the cabin crew reported something hitting the fuselage. They found the fuselage was penetrated and the plane dumped fuel and returned.
In that case the plane had only just left maintenance and was inspected a number of times, but the investigation concluded that only 1 of 13 latches had been closed properly. The probability of missing a countersunk bolthead is lower I guess than missing a latch.
So not stretching the discussion but interested in more facts.