When a commercial airliner disappears from air traffic control RADAR, it first means the aircraft is no longer squawking, meaning the electronics on board are not communicating with the air traffic control system. It does not mean the RADAR stopped seeing a return, a “skin paint” from the aircraft.
It could mean it was turned off and the aircraft diverted.
It could mean the aircraft crashed.
We shall see.
“It could mean it was turned off and the aircraft diverted.”
Interesting point...
That's so 2001 /x
A terrorist need not beat open the cockpit door--he/she/it need only (1) buy a first class ticket, (2) sit close to the flight deck, and (3) wait for one of the pilots to emerge for or return from a potty break. A flight attendant standing guard with galley cart won't stop a fast acting terrorist. Neither would air marshalls--even assuming they're seated nearby.
So let's move beyond the reactive protocol of squawking 7500 (hijack) post facto and employ a new squawk code (7501?) to indicate an authorized yet unsecured flight deck (a flight attendant in the cockpit temporarily is not a secure flight deck). Communicate and confirm with ATC before and afterward of the planned event.
Two things result: (1) ATC will be alerted and can pay special attention to the target on their radar screens and (2) an unauthorized flight deck incursion and/or jeopardy of a crew member can be quickly communicated by having to change only one squawk code digit.
Finally, once the flight deck is re-secured, it's another comm/confirm with ATC before switching back to the originally assigned IFR squawk code.