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.