Newer homes built during the real estate bubble that started around 2002 are built to increasingly stringent energy codes. In states like California, which passed 2010 codes requiring mechanical indoor-outdoor air exchanges, houses have no ventilation.
These homes, left without electricity, and sitting in foreclosure are inevitably going to trap humidity. Trapped humidity is going to breed mold.
An airtight house which can not be infiltrated with fresh air is going to be a perfect breeding ground for mold and mildew.
If you’re in California, you might want to give us a call and see about getting an assessment from Byebyemold.