I've seen flies leave dark spots on lacquered wood, mirrors, glass and monitors. Why do they do that? Is it some food stash or are they dumping waste in a single spot? Why pick a smooth surface?
House flies (Musca domestica) prefer to rest on light colored surfaces, and what is lighter than windows, windowframes, glossy surfaces that reflect light, etc.? Where they rest (not eat) is where they defecate (or regurgitate), leaving marks politely called "fly specks".
House flies also leave dark fecal and regurgitation spots on wall surfaces where they rest, and with a preference for resting on light colored surfaces, these spots can be quite noticeable when fly numbers are high.
If you ever visit an "open" barn, you'll notice the phenomenon is quite remarkable. If the number of flies is high, there will be specks everywhere, but the windows and other shiny surfaces will be more densely covered.