Landing a 777 takes a significant airstrip. Really significant. Once there, it's not simply a taxi to some secluded spot. If your scenario is true, the plane should be visible by satellite.
Well, maybe the highjacker thought that the plane had longer range than it actually had. In that scenario we're looking at painstakingly scanning the ocean with sonar to find the wreckage.
Never going to happen. We don't even have accurate charts for the depth of most of the ocean -- it's just extrapolated from gravity measurements from satellites.
The petroleum industry has some pretty awesome equipment that they use to look for oil and gas. They also maps of much of the sea floor that are way beyond anything that's publicly available. You can certainly find the wreckage given enough time and funding, especially if you can narrow the search area down based on the known facts of the disappearance and create a probability distribution across that search area so that you can search the most probable areas first.