Realize that molds are really just plants, and as plants they need a few things
to grow, such as soil and water. So, since few molds can grow without water,
you must look for damp places where they propagate, such as in bathrooms,
basements, inside walls and ceilings, around plumbing fixtures and pipes, etc.
However, too often that wet place where mold is growing is inside the air
handler and ductwork.
One word of caution here: Some molds are extremely toxic, and
remediation may require the use of medical-quality masks.