ever so slight water ingress
I park my car with the off side wheels on the kurb. After some nights of light rain.I found a damp patch in the passenger side footwell. checked the usual window open, sunroof drain pipe off, leaking heater matrix. none of them.
Audis have a water ingress through the pollen filter bracket A4 2000 on , if I remember. So I checked arround the scuttle trim and found 4 small holes in the trim at both ends. they were crudded up.This I guess alloud water into the foot well through the heater. Cleaned the holes water ingress not returned !
check yours now!