On our Passat Alltrack we have the VW factory fitted retractable towball.
I have noticed that the ball ( with paint removed obviously ) tends to get very rusty in parts when stored,even with an Al-KO rubber towball cover fitted.
I have cleaned the ball up but the marks left by the rust are still just visible but the ball is now OK to use.
Have placed a small polythene bag over the ball secured with the Al-KO towball cover to see if it prevents rust.
My point is that before the first trips of the season it may pay just to have a look at the tow ball if you have this one fitted as it took a little while to clean up which is not what you want to be doing when your nearest and dearest ( or the wife ) is eager to be on their way.