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