How can flies sense mirrors?
It might be pretty straightforward. Common window/mirror glass is opaque to UV, so as far as the fly is concerned mirrors are just translucent surfaces. 

This of course leads to the question of why flies can't see window glass. Maybe the large amount of light passing through at visible frequencies makes it look like a tinted fog to the housefly? Maybe they're trying to fly through mirrors too, but land on their feet and just roll with it?