That musty smell in the back bedroom
You've noticed it by now. A smell in the hallway closet that wasn't there last year. A dark ring on the ceiling drywall in the bathroom that doesn't quite go away when you repaint over it. Maybe a window in the back bedroom that fogs up on the inside every morning, even though nobody's running a shower nearby. In Van Nuys, where the air outside is dry most of the year and the ocean is twenty miles away, this catches people off guard. Dampness is supposed to be a coastal problem, a Ventura or Santa Monica problem. It isn't. It's a Valley problem too, just a quieter one.
Dry air outside does not mean dry walls inside
The San Fernando Valley runs hot and dry for most of the year, then gets a short, intense wet season from December through March. That swing is the whole story. Homes here are built and insulated for heat, not for managing moisture once it gets in. When the rain finally comes, it tends to arrive in a handful of heavy storms rather than steady drizzle, and a lot of older Van Nuys roofs and stucco walls just weren't detailed to shed that kind of water fast.
Add in the irrigation habits of a dry climate. Sprinkler systems running close to the foundation, planter beds built right against stucco, lawns graded so water runs toward the house instead of away from it. None of that is unique to Van Nuys, but it's more common here because people are watering constantly to keep anything green alive, and that water has to go somewhere. Often it goes into the base of an exterior wall, a crawl space, or a slab edge, and just sits there.
More on this from Reading List Why Valley Homes Get Damp Problems Without Any Coastal Humidity.
The housing stock makes it worse
A lot of Van Nuys was built up in the 1950s and 60s, post-war tract homes and low slab-on-grade ranch houses, many without a real crawl space or with one that's barely ventilated. Stucco exteriors from that era were not built with modern moisture barriers behind them. Add decades of patch repairs, satellite dish holes, and re-stuccoed additions, and you get a wall assembly with a dozen small entry points for water that nobody tracked.
Evaporative coolers, once very common in this part of the Valley because of the dry heat, are another factor. They push a steady stream of moist air into the house during the hottest months, and if the ducting or the unit itself has a slow leak, that moisture collects in the attic or around the vents long before anyone notices a stain. Newer AC-only homes don't have this issue, but plenty of older Van Nuys houses still run a swamp cooler as a backup or primary system.
What you can actually check yourself
Start outside. Walk the perimeter of the house after you water the lawn or after a rain, and look for standing puddles against the foundation or stucco that stays dark and wet long after the rest of the yard has dried. Check that sprinkler heads aren't spraying directly onto the wall. Look at your gutters and downspouts, specifically where the downspout empties. If it dumps water two feet from the house instead of carrying it away, that's a fixable problem on a weekend.
Inside, open closets and cabinets on exterior walls and smell them. A flat, earthy smell is worth paying attention to even if you don't see anything. Check under sinks for soft, swollen particleboard. Look at ceiling corners in bathrooms and any room below the attic, especially if you still run a swamp cooler. A flashlight and ten minutes in the attic, looking for dark staining on the underside of the roof deck or on insulation, tells you more than most people expect.
Where it stops being a DIY job
If the smell is there but you can't find a wet spot, or if you find a stain but can't tell whether it's old and dried out or active and growing, that's the point to stop guessing. Mold doesn't always show itself where the moisture actually is. Water can travel along a stud or a pipe and show up as a stain three feet from the leak, and painting over a stain never fixes what's behind the drywall.
A proper mold inspection in a house like this means checking humidity levels room by room, looking behind the obvious spots with a moisture meter, and sometimes pulling a small section of drywall or opening an attic hatch that a homeowner wouldn't normally touch. It's worth doing before a remodel, before buying or selling, or any time the smell has been there for more than a few weeks and you can't pin down why. In a climate like this one, where the damage is usually hidden behind dry-looking walls rather than announced by obvious humidity, that inspection is often the only way to know what you're actually dealing with.