Olgar Shiverstone
Legend
Almost no military tracks commercial flights in that way when the flight is leaving territory and heading out to sea. Just not a concern. Even today NORAD doesn't say anything about a flight heading to say the UK if it alters course on a more Southerly track out of D.C. rather than a Great Circle route.
Well, then what's the Vietnamese military's excuse? The flight was headed into their airspace and dropped off the transponder right before crossing into their airspace. Or Thailand? (I'll neglect the Cambodians -- they don't have a lot of capability). There's a number of countries whose interests overlap in the area where the aircraft disappeared.
I admit I don't know the specific early warning ranges in that part of SE Asia. Ironically we recently completed an exercise focused on the South China Sea and I was looking at radar coverage, but it was in a different part of SCS.