That damp smell that never quite leaves
If you live west of 14th Street, you probably know the smell already. You open a closet that's been shut for a few days and it hits you, something closer to a basement than a bedroom. You run a hand along the bottom of the curtains and they feel cool and faintly tacky, even though it hasn't rained in weeks. In a lot of other cities that would be a sign of a leak. On the westside of Santa Monica it's often just Tuesday.
The culprit is the marine layer, the thick gray fog that rolls in most nights between late spring and early fall and sometimes just parks itself over the coast for days. It doesn't rain, so nobody thinks to check for water damage. But it puts a steady film of moisture on everything, and in an apartment that doesn't get much cross breeze, that moisture has nowhere to go.
Why the building's age and layout matter here
A lot of westside apartment stock, the courtyard buildings and dingbats from the 1950s through the 1970s around Ocean Park, Sunset Park and the areas near Wilshire and Montana, were built for a climate people assumed was dry. Single-pane windows, minimal insulation, bathrooms with no exhaust fan or one that vents into the attic instead of outside. That was fine when summers were the only humid stretch and everyone left windows cracked. It's less fine now that a lot of units have window units or portable ACs running and get sealed up for noise or privacy instead.
Newer construction nearer the Third Street Promenade or along Ocean Avenue handles this better, with real ventilation and tighter building envelopes, but even those buildings get it wrong in the corners facing the water, where fog sits longest and sun rarely reaches the stucco.
More on this from Reading List How Coastal Fog Changes What A Westside Apartment Needs Checking For.
What to actually check yourself
Start with the places that face west or northwest, since that's the side catching the fog and the prevailing onshore flow. Pull furniture a few inches off exterior walls and look behind it. Mold here often shows up as faint gray or black speckling, not the dramatic fuzzy colonies people picture.
Check under sinks and around tub surrounds for a slightly swollen or soft feel in the drywall or baseboard. Look at window sills, especially any that face the ocean, for a chalky white residue, that's often salt left behind after fog evaporates, and underneath it the wood or paint is usually damp longer than you'd expect.
Smell is still one of your better tools. A musty smell in one room that doesn't match the rest of the apartment usually means moisture is sitting somewhere out of sight, inside a wall cavity, under flooring, or above a dropped ceiling tile near a bathroom.
What the fog season does that summer heat elsewhere doesn't
Inland California deals with dry heat that pulls moisture out of buildings. The westside gets the opposite problem for months at a stretch: cool, damp air sitting against stucco and glass overnight, then a short burst of sun that dries the surface but not what's underneath. That cycle, damp all night, dry-looking by noon, is what lets mold get established without anyone noticing a single obvious wet spot. It's not a storm event. It's accumulation.
When it stops being something you can check yourself
If the mold is confined to a small patch, say under a bathroom sink or along a windowsill, and you can trace it to an obvious cause like a slow drip or a window that doesn't seal, that's reasonable to clean and monitor yourself. Standard cleaning with soap and water, drying the area fully, and fixing the source usually handles it.
Where it stops being a do-it-yourself job is when the smell is clearly present but you can't find the source, when discoloration keeps coming back after you clean it, or when it's inside a wall, under flooring, or near HVAC ductwork. At that point you're not dealing with a surface problem anymore, you're dealing with something that needs to be opened up or tested to know how far it's spread. A professional mold inspection at that stage isn't about confirming what you can already see, it's about finding what you can't.