I had this happen on two Audi I own. The rubber inside the struts where it folds develops micro tears and leaks air overtime. I took one strut and inflated it underwater with the dust shield off to see where it leaked from. The rubber usually starts to crack around the 10-12 year mark. So plan on replacing struts around that time in Audis.
Pros: rubber develops micro tears
Cons: leaks air overtime
Vehicle:
Audi
