SamTherapy is right--dirt, dust and liquids (we will NOT go into what types of liquids ;) all tend to accumulate mostly right where the floors and walls meet and in corners. If you do a search in Free Stuff, using ernyoka1 you'll find dozens of trash and litter type props (look at her earliest contributions) that are great to litter urban sets with...and I think I recall a nice graffitti-scrawled brick wall and a section of sidewalk, too.