My take on it--- derived from news reports and experts on CNN, Fox, ABC, NBC.
I haven't heard a single "expert" yet surmise how hijackers could have gotten on board undetected. See item #1 below. Am I the only one that has this theory ??
1) Hijackers avoid going through security by being smuggled into the cargo hold of the plane such as in a large crate or wooden box. Planes routinely carry large crates and freight items. They routinely load heavy things using a forklift. A shipping crate 6 feet square and 6 feet tall would not even be considered that large of an item. None of these hijackers would show up on a passenger list.
2) While down in the cargo hold, they disable and disconnect what communications they can shortly before ascending to the passenger deck. This would explain some of the electronic systems shutting down prior to the pilots' sign-off with air traffic controllers. A trap door in the cabin gives the flight crew access to the lower cargo hold (and the hijackers access to the passenger deck).
3) On the passenger deck, a diversion is created, causing one of the pilots to open the cockpit door---cockpit is taken over by hijackers.
4) Somebody says "good evening" to air traffic control. Nobody knows whom.
5) Transponder is turned off, as well as any other identifiers and tracking devices that can be disabled from cockpit. The plane is essentially flying in stealth / blackout mode at this point. All cell phones and electronic devices are collected from the passengers at gunpoint.
6) The plane is now in control of the hijackers, flying a predetermined route to wherever the destination was. Low altitude flying, zig- zagging, and shadowing other aircraft is used to confuse radar operators and air traffic control. The pilot of the plane is highly experienced and knowledgeable, possibly even ex-military.
7) The plane is landed at an abandoned military base (or similar) and hidden. The U.S. military had bases and outpost all over the place back in the Vietnam and Korea days. Other foreign countries did as well. Many of those installations now sit abandon and forgotten, some of them have also been bought by private individuals and companies. Some of these installations were located on small islands. That particular plane is large in size, but it would be totally possible to land it on a highway or on a short runway with the right person controlling it. Almost any old military or army base is going to have a runway and hangers.
8) If the same people that knew how to turn off communication devices and tracking devices are as capable and proficient as they seem, then they will also know how to remove the black box and get rid of it. This means that many parts of this mystery may NEVER be solved.
9) What use they have for the plane is left to be seen.
10) What use they have for the passengers, if any, is left to be seen.
The fact that this plane vanished isn't the scary part. The scary part is going to be when this plane re-appears.