Even if the military radar DID pick up something unidentified moving west, what would they have been able to work out about it? Presumably there would need to have been some communication with civilian ATC to find out if an aircraft was known to be in the vicinity. Would Malaysian ATC have known at that point that the plane wasn't happily over Vietnam? Is there any established channel of communication between military and civilian structures (the muddled nature of Malaysia's response to this would suggest not)?
Is there any possibility that the low-flying aircraft seen off the east coast of the peninsula were actually fighters, scrambled to intercept an unidentified plane?