So I got in the car this morning and found the rear view mirror glass on the passenger seat, leaving the adhesive on a shiny cellophane-like sheet on the mount. Note that this is not the windscreen attachment, but just the reflective glass bit. For at least a year I've noticed a little marking around the bottom edges a bit like corrosion or something. Must have been detaching glue!
I can get by on wing mirrors for the short term, but I've never seen this happen before and I'm a bit surprised. Car only 3 years old. I presume the warranty claim will be fairly straightforward.