Ok, so I had a new drivers side headlight last week due to a broken internal part that wouldn't let it adjust.

 

I'm guessing this is normal and I've never seen it before as the other unit was faulty, but best check....

 

Sat in stop start traffic on the way home, heading down a slope, I notice the headlight projection on the car in front moving.

As I stop, the drivers side light moves inwards and stays there.

When I reach a speed over about 10mph it moves back out.

Did it repeatedly.  

 

I'm wondering if it moves in as it thinks you're parking or pulling in and tries to blind incoming traffic less?

 

Never seen this before, thought I was imagining it at first!

 

As far as I could tell, the passenger side one was static.

That is correct behaviour of headlights unless LED or halogen