There was a post about this: viewtopic.php?t=21723.
The plastic dust caps have a small hole in the top which allows water to drip into the strut housing - my 2013 did not come with the caps so I bought them. I think from 2013 forward there were no caps so water can collect. On mine the bolts were painted black but after three years it started to wear off. What I did was to plug the small hole in the cap with a small stainless steel screw; I greased the bolts and now no water collects with the caps in place. It's a cheap fix worth doing.