Interesting thing I read earlier today... is that most references in the news to the plane being on "radar" are incorrect. Flight controllers typically don't use radar. They're looking at a screen that is a lot like a radar screen, but is just showing blips that are basically data packets sent by the planes themselves. So if the plane disappears from "radar", all that means is that the blips stopped.
Actual radar systems are only used in military, areas where there is a heavy military/civilian overlap, or in certain areas along the coast of the US.
And even then, if the plane was visible on radar, it would be difficult to isolate it from all of the other unidentified blips and then track it across multiple radar installations. Radar is basically line-of-sight, so once something gets 20+ miles out, your ability to see it is hindered by the height of your radar antenna AGL and the surrounding geography. Edit: and, of course, the elevation of the object AGL. Something that's 5 miles up will be visible from farther away than something that's only at 5k ft.
