“The Radar signature you are referring to wasn’t the Plane’s Transponder signal”
It only takes the turn of one knob to change the transponder signal.
ATC issues the code that you put into the transponder.
You can squak 7500 for hijacking or 7600 for lost communications, all it takes is twisting of the 4 knobs to set any code you want.
Inside job. Co Pilot?