seat belt damp

We had a ton of rain here a few nights ago and I mean it rained hard. I
didn't drive my car until a couple of days later and noticed as I put on
my seat belt that it was cool and damp. Nothing else was wet (seats,
floor) and I opened the sunroof and everything was dry up there too.
It's an 01 330Ci and I have noticed tiny amounts of water getting in the
window when I go through the car wash/dryer. Any idea how the seatbelt
could have gotten wet? Thing is I had a mould problem in my last car
and that's the last thing I want to deal with again.
Thanks,
Jonathan