I don�t know, this is the 21st century, large, searched for things don�t go simply missing anymore..
If a plane breaks up on impact, some floating debris will be found somewhere and with ocean current modeling in combination with other data you should be able to find a rough estimate where to look. The only way to really make it likely to disapear would be to have an unobserved coursechange over the ocean and ditch it intact without anyone opening doors (no emergency slides, life-wests etc...)
If it was in controled flight over the ocean, it more then likely had a destination and more then likely will have reached that. I wonder if they include the glide-ratio into the search, I think a 777 should get at least a glideratio of 20:1 and with max altitude that may have been enough to reach something like jemen, oman or somalia where you might be able to not get discoverd imiately..
The cell phone thing just seems dumb. It was the middle of the night flight and they crew I am sure told everyone to turn their phones off on takeoff then they went outside cell service almost instantly. Of course there is not going to be any cell pings.
and the antena-infrastucture is aimed at the ground, reception up is marginal.. it works sometimes but not reliably. It would also depend on whether the pax would realize something was wrong at what time and if it really was malignous acting, they could have been dead. From poising the onboard food to simply using cabin pressure, there is not really a built in safety against a hostile crew or crewmembers.
