Or maybe it was hijacked and flown until it ran out of fuel and crashed, like Ethiopian flight 961 – but with locked and armoured cockpit doors unlikely – i’m guessing that since 9/11 pilots have strict instructions that in the event of a hijack situation they should ignore the hijackers demands and call in the situation, even if passengers lives are threatened.
Or maybe a suicide attempt by one of the pilots like in Egypt air flight 990. Bump off the other pilot, turn the aircraft around, head out over the Indian Ocean for 4hrs before crashing. Passengers and cabin crew may have been blissfully unaware until the end.
Not sure I believe the conspiracy theory that it’s landed somewhere. Governments are inept and cannot keep secrets for long. Conspiracy theorists always credit government and government institutions with far more capability for keeping things secret that they deserve.