in 1979, A Varig Cargo B707 has vanished not long after its departure from Tokyo in route to LAX. Nothing was ever found, no trace.
In the case of this B777, I would not be surprised if there was a cabin or cargo, fire, the crew being under a high work load and using oxy masks had other priorities and ATC was not the most important. Fire became uncontrollable and the Captain made a successful ditching. Unfortunately, all pax were already dead due to smoke inhalation, and no one evacuated. The aircraft did not brake apart and remained in one piece that is now in the bottom of the ocean. No debris to float. Captain and or FO, could have initially survived.