Thailand, Indonesia, Singapore and Cambodia also in that region that have aircraft capable of doing the air to air bit. If we are talking naval vessels, could have been anyone, but having a surface to air missile capable of hitting an aircraft at cruising altitude? Most terrorist groups seem content with MANPADS (SA-7, Redeye, SA-14, etc) that are way easier to come by and maintain rather than a large complex system.
But even with a potential mishap from a nation state, there still would have had to have been some sort of radio chatter and again, debris that would have/should have been found by now.