Building material that was left in the rain and outside before use can harbor viable mold spores for many years.
Indoor environments are never entirely free of molds.
If buildings are air-conditioned, or windows and doors are kept closed in summer, the number of CFUs within should even be lower than outside levels. It has high moisture requirement, so it grows where moisture has accumulated from roof or wall leaks, or chronically wet areas from plumbing leaks. Possibly co-factor for throat and lung cancers. Some Penicillium species are fairly common indoor fungi, even in clean environments. This particular specie of fungi can proliferate in abundance in indoor environments.