The Sim is built with PBR, Mirrors, and 2K Terrain PBR Textures.  To see everything as it is meant to be seen- including the environmental lighting- please update your Viewer to the latest supported versions be they the official Linden Lab viewer, Firestorm, or others. You will need to turn on advanced lighting settings to see Mirrors correctly.  If you see a shiny, silver surface that looks out of place- that is probably a mirror.