Cotton Hollow is a swimming hole, and the walk back to the car happens in wet clothes. The seat gets a towel, usually. The seat belt does not.
Belt webbing is the single most reliable early warning system in a car interior, and almost nobody looks at it.
Why webbing specifically
A seat belt is dense woven polyester, and it holds a lot of water for its size. When you unclip it, it retracts — wet — into a dark, unventilated cavity inside the B-pillar.
That is a genuinely ideal environment for mildew: damp, dark, no airflow, at cabin temperature, with organic material on the fibre from skin contact. And because most of the belt is inside the pillar, it does not dry between uses.
The visible result is fine grey or black speckling along the webbing, usually appearing first on the section that spends most time retracted. People often assume it is dirt and try to wipe it off. It does not wipe off, because it is growing in the weave.
What it is telling you
By the time you can see it on the belt, the same conditions exist elsewhere. The belt shows first because it is the wettest, darkest, least ventilated textile in the car — not because it is the only affected one.
Check the seat base foam by pressing a knuckle firmly into it, and the carpet under the mats. If the belt is speckled, one of those is usually damp too.
Cleaning it properly
The whole length has to be pulled out and locked or held while it is cleaned, worked along its length, and then — this is the part that gets skipped — allowed to dry fully before it retracts. Letting a freshly cleaned wet belt wind back into the pillar recreates the problem immediately.
That takes time and it is why it gets left out of a quick clean. It is included in our interior detail at $175.
What we will not do
We do not use bleach or solvents on belt webbing, and neither should you.
Seat belt webbing is a load-bearing safety component, and its strength depends on the polyester fibres being intact. Bleach and aggressive solvents degrade those fibres, and the damage is invisible — the belt looks fine and is weaker. There is no way to inspect for it afterwards.
So we clean with appropriate products and we accept that severe, long-established staining may not come out completely. A slightly marked belt that is at full strength is the right trade, and any detailer willing to bleach your belts to make them look better is making a decision they should not be making on your behalf.
If webbing is badly contaminated, replacement through a garage is the correct answer, not chemistry.
Prevention
A towel over the shoulder before you belt up, and leaving belts pulled out to dry after a swimming day. Both free, both effective.