It's literally a locked room mystery. Agatha Christie would start with a dead body in a locked room. There was nothing more tantalizing that being told there was no way for it to happen, while being also told that it did, indeed, happen.
Same with this vanishing plane. It's exactly the type of plane, of all the types of planes, that vannot vanish without a trace. Yet it did.
My other theory is not neraly so FReeper friendly. But if I were an ET civilization, and wanted to get the attention of the world in a non-threatening, but overwhemingly powerful, way, I just might beam up an entire 777 in flight and then at a later date, return it intact and with all passengers safe.
It would rivet the world, and make my point, all at the same time.
Don't tell any other Freeper I said this though - ETs are not popular around here. But if you want to know where I got the idea, watch "The Day the Earth Stood Still."
OK That is just between us!