@amsr - quite a few instances of mirror glass falling out of the door mirror housing have been reported on vwroc.com forum - sometimes while the car is being driven, resulting in the mirror glass hitting the bodywork and causing damage in the process. The general consensus is that regular use of the mirror heating function - or leaving the mirror adjustment control permanently in the heating position - weakens the adhesive over time. IMHO it’s quite easy to unwittingly leave the mirror adjustment in the ‘heating’ position as there’s no warning lamp to show when it’s switched on. So if left switched on, even if it automatically times out after a set time period, the heating will presumably come on with each ignition cycle.
The mirror adjustment / heating control has been the same since at least the mk4 Golf, so maybe it’s time for a redesign, along with the addition of a warning lamp to indicate when the mirror heating function is active.
It might also be worth checking the other door mirror on your car to ensure it‘s still secure and not about to fall out.