[Izzy Dunne]

It is very possible that the hijackers flew along a standard airline route from Singapore to New Delhi where the Indian military air defense radar personnel would ignore the flight as simply a normal airliner, and the Indian civilian air traffic controllers would not even know or “see” the hijacked airliner because the hijackers had turned its transponder off three to five hours earlier.

So... the military will see this blip WITHOUT A TRANSPONDER, and say "Hey, No big deal", while the civilians won't even see it.

Not a good plan for military defense, if you ask me.

[GOPJ]

If the plane was running along established commercial flight routes - and not near a military base.... or flying on the edge of radar from different countries - there's a good they would be missed. No one was looking for something like this to happen... I doubt it could be done today.