It crashed (for whatever reason)
or it didn't.
If it Crashed, either:
Mechanical/electronic failure
Pilot error
Intentional loss of the aircraft, including shoot down.
If it didn't:
Hijacked
Stolen.
Aliens (just kidding).
That the plane crashed, for whatever reason, is possible. If in the ocean, the debris field would have to be located. On land (for instance the central highlands in Nam) the wreckage may be masked by terrain or jungle.
That it was stolen or hijacked, and diverted to another location is also a possibility.
What needs to be examined are the range of the aircraft, and possible destinations.
Bandar Aceh airport has enough runway to land, refuel, and take off fully laden. I would suspect other airfields in range of the aircraft at the time it went off the radar would also be able to provide the same facilities, but the problem is one of getting in and out with a 'hot' plane. This would require cooperation at some level, or some sort of clandestine operation all around.
If a plane that big hit land (crashed) anywhere, it would be smoking for days.
Very strange.