In the Anaheim Colony's century-old bungalows, a musty crawl-space smell almost always traces back to the original wood framing and the galvanized plumbing running underneath it; a slow leak down there can run for years before it ever turns into a stain someone notices. Out in the West Anaheim tracts built during the 1950s and 60s Disneyland boom, the same pattern shows up inside a wall instead of under a floor, because so many of those houses are still on their original cast-iron and galvanized supply lines. In Anaheim Hills, where the lots are newer but the homes run two stories, an upstairs bathroom leak often travels down through a joist bay and turns up as a ceiling stain in the room below, well after the leak itself started.
None of that requires a hidden leak to justify a call, though. If anyone in the house has headaches or allergy symptoms that ease up once they've been away from home for a day or two, that pattern is worth checking out. If you're closing on a house or listing one, a lab-backed inspection gives both sides a document they can actually trust, and if you manage a rental property and a tenant is complaining about the air, the same kind of report replaces an argument with a number. Prior remediation is the other common reason: if a company already treated a property, an inspection is the only real way to confirm it held.