A method for operating an anti-theft device for a motor vehicle which is unlocked when a radio signal is received from a radio-based remote control device of the motor vehicle and, after unlocking, is started by actuating a start/stop operating element. Here, an interior of the motor vehicle is monitored by a detection device of the anti-theft device after the motor vehicle has been started. For the monitoring of the interior of the motor vehicle, a search signal restricted to the interior is output by the detection device in the interior of the motor vehicle. The radio-based remote control device is configured to output a detection signal when the search signal is received. If the detection signal is not received by the detection device before a predetermined search time, a warning message is output to a unit outside the vehicle.