The Transponder goes off
"By signing off from Malaysian airspace at 1.19 a.m. on March 8 with a casual "all right, good night," rather than the crisp radio drill advocated in pilot training, a person now believed to be the co-pilot gave no hint of anything unusual."
" Two minutes later, at 1.21 a.m. local time, the transponder - a device identifying jets to ground controllers - was turned off in a move that experts say could reveal a careful sequence."
And this about the other communication system - ACARS:
"At some point between 1.07 a.m. and 1.37 a.m., investigators believe someone switched off another system called ACARS designed to transmit maintenance data back to the ground.
...
"Cutting the datalink would not have been easy. Instructions are not in the Flight Crew Operating Manual, one pilot said. Whoever did so may have had to climb through a trap door in full view of cabin crew, people familiar with the jet say. Circuit-breakers used to disable the system are in a bay reached through a hatch in the floor next to the lefthand front exit, close to a galley used to prepare meals. Most pilots said it would be impossible to turn off ACARS from inside the cockpit, though two people did not rule it out."
So it is not know which of these systems was shut off first or, in fact, whether they might have gone off simultaneously. And shutting off the ACARS systems would have been or could have been noticed by the flight crew or the passengers.
Alternative conspiracy theory: The ACARS system was rigged in advance with the gadgetry required to do a wireless command to shut it off without going through the hatch on the floor.
Questions, questions, questions.
Maybe HAL took over. Open the pod bay door...