Really weird. I'm thinking along the lines of a rapid decompression at altitude due to some failure in the pressure fuselage. This would cause everybody on board to lose consciousness very quickly succumbing to hypoxia - a lack of oxygen. This may explain why there was no radio comms from the pilots.
The aircraft would just continue flying until it ran out of fuel and eventually just fly into the sea. Although if this was the case, the aircraft's autopilot would have maintained its original flight path details and so it's progress could be monitored and dead reckoning would have it somewhere along it's intended flight path.
Also, when it hit the sea, you would expect it's Emergency Location Beacon to transmit continuously on the emergency distress frequencies which would be picked up by orbiting SARSAT satellites, three fixes of which would be enough to pinpoint a reasonably accurate datum to centre an 'expanding circle' search on.
If it had blown up at altitude, debris would have been spread far and wide. My crew was the first on scene at the terrible Air India 747 terrorist explosion over the Atlantic, west of Ireland in the early 80's which was blown up by Sikh extremist's at altitude. The appalling aftermath was evident all around the ocean, there is a lot of material that doesn't sink, interior panels, soundproofing, foam, rows of seats and apart from the obvious, all their luggage and personal effects contained therein.
It could be that there was a total systems failure on board which caused the autopilot to fail along with a cabin pressure relief valve, causing the decompression and allowing the aircraft to navigate its own path until it hit the water. This may explain why it can't be found as it may be nowhere near where it's expected to be. Although I would still expect the Emergency Locator Beacon to be picked up by satellite.