@Rudedog; If your car is a mk7.5 with LED headlamps (from your signature details it seems to be), then LED’s generate relatively little heat. I think the explanation given in the linked article in @Watts post is a likely cause of dust or condensation inside the headlamps with xenon’s or halogen units which run hotter than LED’s - but less so for LED’s because of their lower operating temperature.
With LED headlamps, IMHO what appears to be dust is more likely to be micro crazing of the polycarbonate lenses, especially if it can only be seen when the lights are illuminated. The headlamp lenses have UV filter protection applied when they’re manufactured and that filter gradually degrades over time as the headlamps get older, resulting in micro crazing. I think the only way it can be rectified is to replace the headlamps; I don’t think it will polish out even if you could get to it, as I believe the micro crazing is in the structure of the polycarbonate rather than on the internal surface.
If headlamp performance isn’t affected, then I wouldn’t worry about it (the polycarbonate lenses on my car’s LED headlamps also show signs of micro crazing when my lights are on. It can’t be seen when the lights are off).
