Here in the U.S., (and I'm assuming it's the same elsewhere, but just explaining for simplicity), cars used to have a simple headlight switch, which also lit up the instrument cluster on the dashboard. It was an easy heuristic: If you can't see the gauges because it's dark, turn on the headlights.

Now, every car has a marketing-gimmick dashboard lit up all the time with all sorts of multi-color lights. In the cars I've been in, the headlight indicator just a small, green light in the corner. Drivers accustomed to the old way think that their headlights are on because the dashboard is lit up. The Toyota Prius was notorious for this when it was new; I used to joke that they didn't come with headlights as a way to save fuel.

It's not as bad now, but people just forget o sometimes. It's worse when cars have day-time running lights, because then the drivers see light coming from the front of the car and think all the marker lights are on.