You may suspect hidden mold if a building smells moldy, but can't see the source, or if you know there has been water damage and residents are reporting health problems. Mold can hide in places such as the back side of drywall, wallpaper, or paneling; the top side of ceiling tiles; or the underside of carpets and pads. Other possible locations of hidden mold include areas inside walls around pipes (with leaking or condensing pipes), the surface of walls behind furniture (where condensation forms), inside ductwork, and in roof materials above ceiling tiles (due to roof leaks or insufficient insulation).