It's hard to say it's been hijacked. It was at 35000ft when it vanished. If you were to have turned off the beacons etc you still have a huge plane to somehow screen from radar. Even in a radar black out area by the time you get below radar the chances are it would have popped back up again? What about some sort of home made emp (or major electrical malfunction), wipe all communications and controls?. Would hit the sea in one piece restricting the debris field? What ever happened they need to find it to find out what the hell happened!