San Diego does not feel like a mold city. Rainfall is low, summers are dry, and most homeowners assume mold is a Pacific Northwest problem. In practice, the coastal strip from Point Loma up through Del Mar carries relative humidity in the 70–85% range for large parts of May through August, and the marine layer holds it there overnight.
Mold does not need liquid water. It needs a surface that stays above roughly 60% surface relative humidity for long enough. In a closed-up coastal bedroom with a north-facing exterior wall, that condition is met almost nightly during a June gloom stretch.
The pattern we find repeatedly: growth on the back of furniture pushed against exterior walls, on closet ceilings, along the base of window jambs and behind headboards. Nothing leaked. The air itself supplied the moisture.
The fix is rarely demolition. It is usually air movement, dehumidification and correcting a specific thermal bridge. But you have to know which wall, which is what moisture metering and thermal imaging are for.