This is genuinely a puzzle.
If a bomb had gone off it would explain the sudden loss of contact, but there would be surface debris scattered far and wide.

However, if there had been engine failure etc the crew would have had a chance to get a mayday out - but this didn't happen either.

I'm thinking aliens.
(Reverend Fister "a disciplined fuckwit", Mon 10 Mar 2014, 9:56, 2 replies, latest was 11 years ago)

Small bomb in the cockpit
Flight controls fucked, comms fucked, nobody flying the plane. Not much debris from that!
(Captain Placid 24/7 ball gags, brownie mix and clown porn, Mon 10 Mar 2014, 10:33, Reply)

The US Navy reckon they can find debris the size of a basketball
Are you telling me that a plane falling out of the sky and hitting the water created no debris?
(Reverend Fister "a disciplined fuckwit", Mon 10 Mar 2014, 11:29, Reply)

There may not be much that floats

(Captain Placid 24/7 ball gags, brownie mix and clown porn, Mon 10 Mar 2014, 11:33, Reply)

North Korean missile test

( The Light in Chains don't touch the Pope's boner, Mon 10 Mar 2014, 10:34, Reply)