I worked in the communication section of Civil aviatioin dept. I find something is being told incorrect. They say transponder was made switchecd off so Radar could not detect the plane any more . Ground Radar works independently where reflected signal from aircraft metal body is received by the ground Radar and ATC knows it's position. Then how the aircraft went missing from Radar screen is not clear unless terrorists used Jammer. Something the authority hiding I feel.
Unless transponder does not emit the response signal, the corresponding flight details can no be recognised.
What u r talking is the primary radars which can not be used effectively for long range. The flight can be tracked with the primary radar only over limited range or with the help of other primary radar in the flight path. But still there will be transponder which works on relieving the signal & transmit it back at pre-designated frequency levels.
Recently there was a confusion in Mumbai Air traffic control since Emirates airliner used the wrong frequency & there was a probability of accident.