what is a “stolen transponder code”?
There is no such thing. The transponder is a device that is manually set with a code by the pilot at the request and direction of air traffic control. In general aviation it is a 4 digit code. It is the equivalent of a number being assigned to a marathon runner you wear during the race. The transponder ("mode C" transponders) broadcasts airspeed and altitude and that information can then be tracked along with the position (from radar) by ATC.