Mid-air explosions actually have a very significant radar signature, called 'bloom', as the debris field expands. If your theory is that the aircraft exploded

Mid-air explosions actually have a very significant radar signature, called 'bloom', as the debris field expands. If your theory is that the aircraft exploded somewhere between Malaysia and Vietnam then it would have appeared as a rapidly-increasing radar return even on civilian radars. Source: personal anecdote from ex-mil radar operator, who also noted that it was possible to track the larger pieces of debris from the target drones and direct the following-up missiles towards them