A transponder system for a motor vehicle includes a transponder that transmits an access signal as a response to a query signal, and a signal processing device that transmits the query signal and receives the access signal, which the transponder transmits in response to the query signal. The signal processing device transmits the query signal as a transponder check signal if the motor vehicle is deactivated, a warning signal being generated if the access signal is received in response to the transponder check signal.