The dusk sensor detects light levels and automatically turns on your headlights when light is low, helping you to drive more safely. The dusk sensor system is fitted behind your internal mirror. It will turn on the dipped headlights whenever conditions demand, such as when you are driving through a tunnel, for example. Your headlights must be switched to 'auto' for the dusk sensors to work. They turn off automatically once you're back in the light.

Article source: www.volkswagen.co.uk