Air interdiction
To the guys with the shoot down theory:
Hamster3null: Modern air force now SHOULD have their procedures squared away now. Sure, they can't protect everywhere in a big country but they will have their procedures oiled out to protect the really sensitive areas. Protocols are in place for all types of timelines.
Onetrack: Totally agree with you
I suspect the simple answer to the RMAF angle is they simply do not know. Perhaps they have a scheduled maintenance of the radar, perhaps they have an operator who forgot to turn on the 'recording' function, perhaps there is just a gap in their radar in that region. Again, not that Malaysia, Thailand and Vietnam are expecting some conflict of any sort requiring a high level of surveillance. However, it would be too embarrassing to admit to it.
Sometimes the most likely answer is the simplest answer. They simply do not know. Just check out what the Pakistan Chief of Air Force said when the Air Force failed to detect the helicopters going in to get OBL.