2 things I am sure are happening.
1) The RSAF is studying tapes from its Gulfstream GS550 AEW flights for that night. The RSAF keeps round the clock aerial surveillance on Malaysian airspace with these GS550s, especially over the RMAF primary airfields. I am sure the radar on this plane also picks up low level aircraft in case the 777 decided to fly at sea level to avoid ground based radar.
2) If the RSAF has radar tapes on this flight, I don't think they will reveal it to the Malaysians for fear of a diplomatic incident.