Some car thieves have learned to steal certain models of cars that use radio signals for keyless entry. They are using a device that amplifies the signal on keyless fobs, tricking the car into unlocking the door and starting the motor.
You can prevent thieves from amplifying the signal from your keyless fob by enclosing it in metal. Some people are using metal boxes. Others are using aluminum foil. Still others are putting their keyless fobs in the refrigerator or freezer to prevent signal amplification. Learn more by reading this article from Technically DC.