Logical speculation would indicate this was a hijacking. Transponders turned off, low altitude flying to avoid radar. Perhaps the hijackers got disoriented flying for hours over water in darkness and crashed before reaching intended target / destination.
Landing a big airplane like a 777 would be hard to keep secret.
Maybe Laos ?
It's hard to imagine that a plane that large crashed without their being something on satellite photos or other data. Without any evidence of a crash showing up yet, one has to wonder if that plane was successfully landed at an abandoned airfield.
It's all too weird.