Wow, Never thought zaino zcs would be the product to solve this, but that is my rake away from reading this. I have zcs, so that would be doable. I think it is a silicone product but not sure. Probably a silicone spray/liquid of another sort would work too.
Mine is a 2011 and does this intermittently with temperature and road surface. Dealer of course couldn't replicate but honestly I can't always either.
I too think it is the rear cover piece.
Glad I found this thread. I'll have to try zcs sometime. Did anyone apply it in a cleaner manner than spraying it as a mist?