according to wikipedia there are 2 types of radar,
Primary- It is a passive radar that senses aircraft by pinging radio waves off the skin of the airplane.
Secondary Radar- which when it receives the primary ping, it responds with information about the plane.
If you shut off the transponder, i think you would deactivate the secondary radar, but the primary is passive that cannot be controlled from the plane. Im sure that they are researching if there were any radar hits that they could not id which plane it was.
The question is, is it possible that you can fly for awhile without any form of radar hit.... could they have gone "off the grid". I personally doubt it. But i have no clue of the range of radar etc.
There is Civilian Air Traffic Control Radar and Military Radar which operates very differently.
Civilian ATC heavily relies on the Aircraft's Transponder, each plane is assigned one by ATC and/or ADS-B which is more advanced than just the Aircraft Transponder. The Airline can also communicate and track the aircraft via their ACARS system which is in constant communication with the aircraft.
An Aircraft can turn off its Transponder and ADS-B and it will be difficult for ATC radar to track but not impossible. Military Radar would have no problem tracking the aircraft especially if it was a Target Acquisition/Tracking type of Radar where they can get all the information using Radar alone that civilian ATC needs the Transponder on for that info.
The area where the aircraft was flying is saturated by air defense radar. The South China Sea is extremely tense with territorial disputes and countries in that area have extremely powerful military radar that I am sure they are all playing back to see if they can track the aircraft.