I did a similar test when returning from the allowed daily walk... agreed it isn't that.
May be the dash sensor doubles up for the lights on sensor.... so its not that.
I did have an idea that it was the rain sensor and yes it is. Those are the infrared sender beams and the slot looks up at what it reflects on. Probably if it's like the infotainment screen you'd see them in a digital picture. Often I get the hand sensor beams on photos.
Here we go:
https://www.volkswagen.co.uk/technology/comfort/auto-rain-sensors
The sensor system consists of a number of infrared light-emitting diodes (LEDs) and a central photodiode. The light emitted by the LEDs is reflected by the windscreen onto the photo sensor. The more drops of water there are on the windscreen, the less light the sensor receives. This information is passed to an electronic control unit, which adapts the wipe intervals of the windscreen wiper accordingly.
If you take a picture of the front of the car I reckon you will see them, but that's what they are... I reckon although the text says less and more. I would have thought it was seeing more reflection from the outside on the beam. If you dont read it like that then the photocell beam and sensor must been contained in sealed windscreen slot. Reckon that is just the camera that is looking up at illuminated spots.... as we all head outside with digital cameras
(photo didn't see anything doesn't mean it's not there tho). Let you do that. Always try the watering can trick with the hand over that area.
A wiki provides the physics of how it works under 'physics of rain sensor'. The refractive index changes when water is on the outside of the glass.
https://en.m.wikipedia.org/wiki/Rain_sensor
We know that, but whether that is the illuminating beam that is used... feels a bit not quite right looking at it and it's all contained within the glass sensor area or whether that is some adjunct to the system we are looking at the degree of rain rather than presence of rain. I'm not sure the average dealer would know. Probably not totally answered the question but we have the physics of how the rain sensor works
.