Most people seems to be assuming that the a/c ran out of fuel then speared into the ocean.
What if all the turns it made were to lose time and reduce the weight of the a/c to a minimum before attempting a ditching over the deepest part of the indian ocean, where a boat was waiting for them. All the pax and crew would already be dead or hypoxic, leaving only the high jackers to evacuate.
If it were terrorists, they could then carry out the same again at a later date, having now gained even more knowledge from the SAR attempt and this forum.
If the ditching was done with the outflow valves open I'm sure it would soon fill with water and sink leaving no trace.