As easily as people go missing in a city. The world is a pretty large place. There are plenty of aircraft flitting here and there every hour of every day. Fire probably incapacitate the crew or decompression and interrupted the transponder.
But with so many spy satellites (may be hundreds in number) around the world, is it believable that none of them spotted an object as big as this plane?
You don't get 100% coverage of the Earth and spy sats and most are pointed at important things like your backyard or the people that are hiding things they are not trained on civilian flight routes.
You do know the relative size of the Earth to a plane and the coverage a small number of Sats have right? You know that the field of view is also VERY VERY narrow to get hubble like resolution on the ground. To see a PENNY on the ground is one thing when it doesn't move and the sat is whizzing by. Now you want to track a plane that was not on a planned flight route changing altitude etc. Much much harder to find a plane in the sky sans transponder than you think... guess why they have transponders?
