The nearest suitable airport at the point where they made their left turn was Sultan Mahmud (TGG), on the north coast of the Malaysian Peninsula. It has an 11,417 foot asphalt runway perpendicular to the shore. If there had been a fire on board, they should have made a U-turn and headed there, a distance of 111 statute miles. We also should have heard a mayday.
I don’t know much about this mystery, so I will leave it to you.