Under an older house
Not everything here is on a slab. The pre-war neighbourhoods — north Inglewood, Magnolia Park in Burbank, Uptown Whittier, the older streets of Long Beach — sit on raised foundations with a crawl space underneath, and that space changes both how a leak behaves and how it is found.
The advantage nobody uses
The underside of the floor can be inspected. That sounds obvious and it is genuinely valuable: a leak that would be invisible under a continuous slab can be located in an afternoon by someone willing to go under the house with a light and a meter. Repairs are reachable without cutting into finished rooms. Drying can often be done from below, which leaves the interior intact.
The reason this advantage goes unused is simply that almost nobody goes down there. A crawl space is inspected when a house is sold and then, in many cases, not again for a decade.
What the space does with moisture
A crawl space with a bare earth floor releases moisture upward continuously. Under normal conditions the ventilation handles it. Add a slow plumbing leak, or irrigation soaking the perimeter for most of a rainless year, and the space becomes a reservoir sitting directly beneath a finished house.
The material that suffers first is not the framing but the insulation and the subfloor. Batt insulation between joists absorbs and holds, sagging away from the floor above it. The subfloor takes on moisture from below where nothing is finished and nothing is sealed. The first sign upstairs is usually a floor that has begun to move, or a smell in one room that does not have an obvious source in that room.
Ductwork and plumbing sharing the space
In many of these houses the ducts run under the floor rather than through an attic. Metal duct in a damp crawl space sweats, and the condensation it produces is easy to mistake for a plumbing leak and vice versa. Establishing which one is happening changes the repair entirely, and it is done by looking at where the moisture is coldest rather than where it is wettest.