What is this parts stock behind rear driver side fender liner?
 

I replaced both rear shocks due to leak yesterday. When I removed the rear driver side fender wall (the black fender liner), I saw something looks like a small filter connects to a tube behind it. There is none by the rear passanger side.

Just wondering what is that for? Sometimes German engineering surprises me...

a pic would help

Don't have my digit cam while doing the job and my Blackberry doesn't have a digit cam either. It's too late now. I already put the liner and wheel back.