The Left4Dead.txt info file says something about the optional objectives. They may appear as "incomplete" (red circle with one diagonal line through...
Indeed, it's a shame that the map of the City in the Keeper library in TDP and the one given to us in DS are in typical Thief-like fashion very vague...
Years ago when all these features first dropped, I tried using the incidence funcs and multiple texture passes to simulate sparkle on snowy terrain....