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?
Now the tinfoil hat guy ideas are done... how about the reality. 14000 ft down in the Indian Ocean... good luck finding that airplane quickly spread out over 10kms give it could have broken up well above the surface of the water. There are plenty of examples of this sort of catastrophe and it took months and years to find the cause and the planes. The simple fact is planes are pretty safe but even the best engineered ones in the world could have failures as a result of poor maintenance or even some small fabrication error or repeated abuse over it's lifetime. It could have been anything other than some sort of tinfoil hat conspiracy. It will likely be some sort of depressurization even / fire that incapacitated the pilots and allowed the plane to fly on Autopilot.
http://en.wikipedia.org/wiki/China_Airlines_Flight_611http://www.casa.gov.au/wcmswr/_assets/main/fsa/2005/aug/28-33.pdfhttp://en.wikipedia.org/wiki/Air_France_Flight_447http://en.wikipedia.org/wiki/1999_South_Dakota_Learjet_crashYou don't have to make shit up to figure out what really caused this accident. Likely when they get black box and recover the pieces off the ocean floor and check out the maintenance record something will pop out very clearly on this. It ain't at diego garcia.
http://en.wikipedia.org/wiki/Uncontrolled_decompression#Notable_decompression_accidents_and_incidentsHere is a miracle event of a plane on fire and the crew saved the flight and my family knows the Captain.
http://en.wikipedia.org/wiki/Air_Canada_Flight_797