> I'd rate the ability of the US Gov't to successfully keep something like that a secret to be roughly zero, plus or minus zero.
You'd be very wrong. There are hundreds (thousands? tens of thousands?) classified missions going on all the time involving large amount of people. And aside for the exceedingly rare whistleblower, all of them are kept secret for decades or more until they get declassified (sometimes never).
(Not trying to give any credibility to the previous comment, just commenting on how effectively governments keep classified secrets.)
As I said, 'like that.' It is difficult to imagine a reasonable comparison between all of the mundane secret missions going on every day, and shooting down an airliner. This is on the level of 9/11 conspiracies. It's just not possible for something that huge to stay secret for long.
You can be reasonably confident that the US does not regularly disappear entire airplanes filled with foreign nationals. As evidenced by MH 370, it's something that everybody notices.
The US has vast intelligence and military resources, but resources can't create operational miracles. Disappearing a civilian airplane and keeping it disappeared would be such a miracle.