My front left air strut was leaking which eventually resulted in a hard fault which I had to have cleared with a T4. I replaced the strut and took it apart to learn what the issue had been. What threw me was the truck would sit at offroad height for days without leaking, it only leaked at "highway" settings and when the wheel was pushed up or loaded in a corner.

The problem was caused by cracks in the rubber where the airbag rolls over on its self at lower height settings. Here is a picture of the problem.

Click image to enlarge