I've just had this happen to me tonight with my 2013 500S. It's been in the dealers overnight for break cylinders being replaced (wasn't done during service a month ago as a part had seized and they didn't have the part) but on collecting it tonight the first thing my wife has seen opening the door to get in is this lump. My pic is almost identical to OPs.
I reported it to my rep and they took pics, but I don't think they are going to do anything about it on an 8yo car out of warranty.