Pretty much only two reasons why a plane loses contact. Mechanical/electrical failure, or deliberately done. If it was mechanical/electrical, the plane probably would have smashed in to the ocean, and there would be debris everywhere to be seen. Unless it was electrical, and the pilot managed to ditch it without breaking up (ala Miracle on the Hudson), then it sunk in one piece leaving little to no debris on the surface.
If it was a deliberate act, then it doesn't make sense to me to fly the thing west to the Indian Ocean and ditch it there. Unless the hijacking went wrong, but then it would have smashed in to the surface and leave a debris trail. And to land it, you'd need a fairly large runway. This isn't some Piper Tomahawk that you could land on a beach in 73 feet.