The engines of MH370 were pinging until 8:11am, which is being assumed as the spashdown. I don't think Malaysia has a fighter that can fly 4000 miles without refueling, and, if they had flown any of their fighters, there would have been identifying handshakes on military frequencies popping up by now, so I think Malaysia splashing their own plane is pretty much off the table as a possibility.
OK, my theory 2 - In Pakistain or Iran getting refitted for a flight back to... ?