or the plane didn't go down? maybe that was why no one saw it or heard it ..
The onboard computer network of the 777 was found to be vulnerable to hacking. In August of 2012, Boeing put in a request from the FAA to modify it to make it less vulnerable. The passenger network was connected to the avionic network - a passenger could have taken control of the plane's avionics controls. The approval to correct this has not been approved to this date.
In mid-air, it is possible it could have been hijacked, and switched their 4 digit flight number with any other flight that was nearby ... which would have camoflauged them to radio control towers. In other words, they could have taken the flight "identity" of any other nearby flight, without any radio control radar noticing. Especially in the vicinity of well known routes ... they could have virtually disappeared after turning the transponder off.
Highly unlikely but possible ..