An apparatus for reminding a user that there is a subject in a vehicle includes a sensor, an warning device, an electrical connector configured to couple to a cigarette adaptor outlet in a vehicle for receiving power therefrom, and a circuit coupled to the electrical connector, wherein the circuit is configured to activate the warning device when the sensor senses a subject in a vehicle, and when there is a lack of power from the cigarette adaptor outlet. A method of operating a warning device of a system that is configured to sense a subject in a vehicle, includes sensing a lack of power from a cigarette adaptor outlet in the vehicle, sensing the subject in the vehicle, and activating the warning device in response to the lack of power from the cigarette adaptor outlet in the vehicle and the sensed subject in the vehicle.