There are so many things that could have happened, that until they start coming up with some wreckage, no one will really know.
They are searching in a massive area, its like trying to find a Mini car in England, without any help as to where it may be.
Possible Options:
1. In air break up, in area not yet explored. Expect wreckage to start appear on beaches etc after 5 to 7 days +.
2. Hijack and landed somewhere. Possible, in that the transponder can be turned off, and radar in that area isn't as sensitive as land / populated areas.
3. Water landing, complete, like what happened on the Hudson River a few years ago. The plane would sink, in tact. There would be little or no debris, and if passengers did escape, they would be tiny specs, and would need a rescue craft to fly virtually directly overhead to be seen.
4. A TV company is starting a new show 'Lost: With real people' on some pacific island.
I'm sure in a week or two, we'll start to know a bit more.