The marine layer and the cold surface
Some of the moisture found in houses along this coast never came out of a pipe. It came out of the air, and it chose the coldest surface available. Telling the two apart is the difference between a plumbing repair and a ventilation problem, and they are confused often enough to be worth a page.
Why the coast behaves differently
Through late spring and early summer, the overnight marine layer keeps humidity high in the coastal cities on this site — Manhattan Beach, Redondo, El Segundo, the shoreline side of Long Beach, and to a lesser degree Culver City and Hawthorne. Afternoons burn off and dry out. The result is a daily swing, and every swing is an opportunity for moisture to condense on whatever surface sits at the cold end of it.
Camarillo has the sharpest version of this on the whole site, because the fog reaches it off the Oxnard plain in the morning while the afternoon runs genuinely hot. Simi Valley, on the other side of the hills, has almost none of it — which is why the same building problem does not occur there.
Where it shows up
An uninsulated duct running through a crawl space or an attic. The underside of a floor over a ventilated crawl space. The inside face of an exterior wall in a room that is closed up and unheated. The back of a closet on a north-facing wall. Around a window frame where the frame conducts and the wall does not.
The tell is location rather than quantity. Condensation appears on cold surfaces regardless of what is above them; leakage appears below something that carries water. If there is no plumbing anywhere above a damp patch, the explanation is usually in the air.
Why it matters for drying, not just for diagnosis
A house that is genuinely wet from a leak still has to give its moisture back to the air, and on this coast the air is often unwilling to take it. That is the practical reason drying schedules here run longer than the same job inland, and the reason opening the windows on a June morning in the Sand Section makes things worse rather than better.
It is also the mechanism behind the long delay between a water event and the first sign of growth in these cities. The wall was closed while it was still above its reference reading, the coastal air gave it no help, and the consequence arrived months later in a season nobody connects to it.