2014 A5 Premium Plus Cabriolet Quattro 2.0T
So, apparently after 9 years or so, this can happen. The button itself is about as mobile as a tiddly-wink but luckily I felt it down on the floor soon after I started looking. The rubber snubber doesn't move around as easily; seeing it sitting loose next to the shift quadrant was the observation that started my search. I'm not sure what I did to cause the button to come off, and I'm not sure whether anything cracked or broke; maybe I just put a little more "English" on it than usual when pushing it and off it popped. I do know that replacing the stereo volume knob, barely bigger than a pencil eraser, cost me about $100 five years ago, so I'm crossing my fingers this new issue does not recur.