What is the first thing you do when you land? I don't know about you, but I check my phone. If even one phone on that plane was not off or in airplane mode it would immediately connect with the nearest cell tower no matter where they landed, if there was a cell tower in range that is. I know for a fact that I've simply forgotten to turn my phone off... on occasion... and sure enough, my phone starts ringing as soon as I land.
Now, who here would say that the NSA isn't backdoored into every cell tower on that planet. I wouldn't be surprised if they are. How could we not know as soon as one of the cells on that plane popped up on a tower anywhere in the world? So, exclude all cell tower coverage areas in projected flight area and search there to rule out potential hijack/landing.
Most people still turn their phones off before a flight to save power or because they think it will keep them safe. If they are dead, they can't turn the phones back on and the hijackers can go around searching for phones if they really want to. So it's pretty unlikely that phones will attempt to make contact with cell towers.
But assuming that a phone does make contact, there would be massive cooperation required to detect it. The airline and victims' families would have to give a list of phone numbers that could be on the flight, and all the carriers in the region would search through their towers' logs to see if any of those IMEIs (assuming they keep those logs for at least a week).
All it takes is one phone to be on, regardless of human intervention. I would wager someone forgot to turn their phone off. Nevertheless, if the flight was hijacked and the hijackers were pros, as they clearly must be, they might have had a cell jammer. Regarding IMEIs, cell numbers of passengers, and tower logs, well that's what we have all those 3 letter acronym agencies for. I'm sure they haven't just been sitting around all this time. I don't think it's just the navy and air force sending search and rescue.
These guys were also incredibly prepared it seems. I would suggest that it would be unlikely that passengers, if they were still alive, would be allowed to just move about and grab things. They could be blindfolded and told not to move, for example, during landing, refueling, and so forth (though blindfolds may not be as relevant at night.