I had this happen to my old Mk5, the glass mirror plate had separated from the plastic mount and was slowly pulling away. It got worse as time went on to the point that if you accelorated hard the mirror would tip as it was only stuck on at the top. When i explained the fault to vw they said that it is a one part peice, as in it is mirror glass with mount attached, not mirror glass + mount. Unfortunatly i didn't find a permanent fix for it as i got rid of the car soon after so some reg plate double sided tape held it in place temporarily.
If you can get your fingers/something behind the glass, just see if it is pulling away, otherwise i would suspect the mount is the issue.
Sorry i can't help with a fix.