Sounds like it is more a question of paint fading vs. dirt.
"Weathering" doesn't just mean making things dirty.....the lack of visible lettering might be due to the car not having been relettered/repainted for
some time. If the car was filthy it would likely show up somehow (especially around the doors/vestibules where people rub up against the car getting
on and off) and on the ends where the wheels kick up road grime. Even a layer of soot / coal dust would have some variation to tip off that it is dirty.
My guess would be fading vs. dirt if you don't see any evidence that the car is filthy or has grime in some areas....
Charlie Vlk