I think the most likely air defense unit that would shoot it down is Malaysia’s. They should have scrambled, but they claim they didn’t.
The one common characteristic of post-Cold War military budgets in the region is they have nose-dived even more than in Europe. These guys have zero readiness. Given how bad people are about paperwork there, I expect most unidentified aircraft are simply noted down in a log. And justifiably so. It's not as if there's any impending war on the horizon. North Korea's the only country in East Asia that might shoot down an unidentified aircraft.
Another possibility, of course, is that the plane was brought down during an air defense exercise, as occurred in Ukraine back in 2001. In that event, a cover-up would be understandable, no matter which military was involved - it's a major black eye. The problem with this hypothesis (and any hypothesis about military involvement) is that sigint, satellite recon or other intel from one of the big 3 (US, China or Russia) would presumably have noted the missile launch. The cause of the 2001 incident came to light after Uncle Sam said it was the result of a Ukrainian SAM launch.
The comment about reduced military budgets relate all countries in the region with the exception of China, which has seen its budget skyrocket along with its GDP, such that the present number is about $160b.
Malaysian Air Force was first to say they tracked the plane.....then they retracted that......then said it was true.
So far, just about everything the Malaysian government has said has turned out to be lies, including the words “and” and “the.”
Early TV coverage reported the airliner had a plane closely following it. So something was scrambled or diverted to check it out. Perhaps just a phantom if that is possible because it hasn’t been mentioned since but I don’t watch much TV. Sorry no link or other details.