Radar Data from Neighboring Countries (Civilian & Military)?
There is a strange gap in news coverage relating to primary and "secondary" radar data collected by neighboring countries.
If the aircraft did indeed cross the northern part of peninsular Malaysia, civilian and military radar stations in Thailand should also have spotted the aircraft.
Similarly, if the aircraft proceeded westward, it should have been picked up by civilian and military radar in Indonesia (Sumatra).
Lastly, India is likely to have significant military radar assets on Great Nicobar island and certainly on the Andamans.
Some of the military radar facilities (particularly India's) are likely to have over-the-horizon capability, i.e. they would be able to spot aircraft over a range of perhaps more than 1,000nm, although with rather degraded accuracy.