car symbol light is on when the car is turned off

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Hi. I have a 2006 Toyota Prius. There is a blinking light on the dashboard when the engine is off. The light turns off when the car is started. The light is a car symbol with a vertical line intersecting the roof of the car. The manual indicates this light relates to the electrical system. The manual says to stop the car immedicately if the light comes on during operation of the car, but does not say what to do if the light comes on when the car is turned off. Is this a reminder of a need for regular servicing, or an indication of a problem?