Mold needs 3 things to grow:
1)Water source: This can be an over flowed toilet, leak in a water supply, malfunctioning AC unit pumping high humidity into the home, ground water from rain storms, storm damage, window leaks and so forth. Home maintenance is critical to prevent excessive water into a home.
2) Time to grow: Remember mold is a plant. Therefore, it needs time to grow. You will most likely smell it prior to seeing it. Fungal growth is a microscopic plant so it needs time to grow to become visible. On average the mold would be visible around 7-11 days.
3)Food source: Any kind of cellulose material is a food source for mold. For example, paper, wood, glue, and leathers. You can look around your home and see so many examples of these.