You never see mold growing on bright, sun-lit surfaces. Dusty or musty papers, cardboard, carpet, upholstered furniture and bedding. Damp wood, such as house framing wet from rain during construction, or for that matter, before the contruction even began. Use kiln-dried lumber if you are building. Air-conditioner insulation, coils, and drain pans, even in furnaces and automobiles.
Poor filters can even grow mold.
Leaks anywhere in the system are sure-fire mold growers. Drain pans under frost free refrigerators. Clogged drain pans in air systems frequently result in leaks into walls and adjacent areas. Stachybotyrs atra growth behind the wallpaper was determined to be the cause. In February 2000, an entire Elementary School was closed and all the students transferred when an overgrowth of Stachybotyrs atra mold was identified growing throughout the area.