Mould that keeps returning to the same spot is the clearest sign you need more than a spray bottle. Either what’s growing underneath was never touched, or something is still feeding it — a slow leak, condensation buildup, a plumbing fitting that weeps a little every time someone showers. You also need help when the area is bigger than a couple square feet, when it’s gotten into drywall, ceiling tile, wood, or carpet, or when you’re catching that musty smell but can’t figure out where it’s actually coming from.
Santa Ana has its own set of conditions that push this problem along. The marine layer that rolls in off the Pacific keeps ambient humidity elevated even on days that feel dry, and a lot of the housing stock here — particularly the older single-family homes and apartment complexes built in the 1950s through 1970s — wasn’t designed with that in mind. Slab foundations with no crawlspace, low-slope rooflines, and minimal attic ventilation mean moisture has nowhere to go. Toss in the seasonal rains and the condensation that collects where cool wall surfaces meet warm interior air, and you get a situation where mould can quietly build for months before anyone notices it. The spots it likes best tend to be the ones you’re not checking: inside closets, behind furniture sitting against exterior walls, under bathroom vanities, inside the cabinets near the kitchen sink. By the time you’ve been cleaning the same corner three times, you’re almost certainly dealing with more than what you can see.