As somebody who's had his rear view mirror messed with a lot recently, I have a guess. There is an area in the cabin ceiling lining where all the wiring for the mirror goes. If you feel the edge of the lining in front of the mirror at the top of the windshield, you'll find a "gap" that is open to the (presumably) warm air from your cabin. So, that spot may be relatively less well insulated between the cabin and the outside air ... allowing that region of the outside roof to warm enough to melt the ice ... maybe?
Just saw this. Another, crazy possibility is, do you park in the same parking space everyday? Are there mirrored or glass buildings around your parking space? I read an article about someone that parked in front of a mirrored building and the mirrors reflective properties and the sun was like a magnifying glass on their car and was melting the exterior plastic. Just a thought.