My Dad's old C200k used to do that. Lack of use was his cause, but repeatedly letting go of the handbrake lever eased it off. If the light is still showing, the brake is not totally released. I believe it can be adjusted and lubricated from under the rear seat.
The pedal isn't returning fully to its rest position, a fairly common complaint. You can make the light go out by hooking your foot under the pedal and pulling it up fully while the release handle is pulled.