If there had been a fault there would have been a distress call.
If there had been a hijacking there would have been phone calls. Unless it was hijacked at a high enough altitude for phones not to work and they managed to get all the phones off people before reaching cell phone range. But its unlikely that someone didnt sneakily keep their phone and text later......
So either there was a sudden explosion and no time to mayday call. Or the thing where there's gradual decompression and everyone falls asleep happens. But then why didnt the radar track it as the plane would have carried on flying for hours??
Possibility that the pilot was in on it, flew the plane somewhere else and passengers didnt realise they'd been hijacked. So no phone calls....they land somewhere out of cell phone range?? North Korea, The Yemen. But again why no radar tracking?
So I do think that a sudden and catastrophic incident such as an explosion is the most likely explanation. Just can't understand why they can't find it.