OK. It's a bit of a thread hijack, but what the hell ...
Bob (my brother) said it was most likely a combination of two things. Failure of cabin pressurisation coupled with failure of the crew's oxygen supply. Ghost plane scenario.
Loss of pressure in the emergency oxygen cylinders is not un common, and it brings up an alarm. The normal response by crew is to disable the alarm so it doesn't annoy the crap out of them during their flight. However, if they forget to report it, the oxygen system stays empty and the next crew may not notice because the alarm has been disabled. It could stay like that for any number of subsequent flights until someone notices.
So, they're flying along and there's a loss of cabin pressure. Not an explosive decompression because there was no Mayday reported. So was probably a malfunction in the pressurisation system. A low cabin pressure alarm would have gone off, but that's not panic stations. The crew would calmly put on their oxygen masks and start to go through the response checklist for low cabin pressure. First thing on the list is to switch off the auto-pilot and prepare to descend to a level where oxygen is not required. Apparently there's a bunch of stuff to do before they actually descend, so it takes a few minutes. But if they had no oxygen in their emergency supply, the crew may have passed out before they got very far through the list.
What we end up with is a plane full of unconscious people flying along without autopilot engaged. Boeing aircraft are pretty stable and will generally maintain their altitude and direction when flying level with no control inputs. So it would have kept flying subject to the vagaries of whatever wind streams they happen to fly through until it ran out of fuel, apparently somewhere over the Indian Ocean.
There are a bunch of conspiracy theories, hijacking theories, deliberate ditching by the crew theories, etc ... But if anything untoward had happened on board, the passengers would have become Distressed. Agitated or upset passengers who think they're about to die get their phones out and start ringing loved ones. If there'd been any commotion on board, mobile phones would have started ringing all over the place. But there was nothing. The plane just went quiet. Everyone went to sleep without any panic or distress.
what's the theory?? ???