Embarrassing or what. And no, it's not the little sun visor behind the rear view mirror.
After 7.5 years of S3 ownership I've only just accidentally found out when the centre button of the key fob does. When I bought the car,I assumed it would allow you to open just the boot when the rest of the car is locked. I tried it and it didn't work, assumed mechanism was jammed, so gave up.
Tonight I got in the car and accidentally pressed the middle button on the fob - and the boot popped. So now know you have to unlock the car first before you pop the boot.
RTFM springs to mind....