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It could be on the seat aswell, but you wouldnt notice it as the seat can absorbe more before it was noticably wet. Also check for any damage on the water runner things on the roof. Also think about removing the panel that covers where the seatbelt joins the frame. Check your window seals as it may be leaking in then just slowly making its way to your seatbelt (behind the panel) and then getting absorbed in. Dont forget water can travel against gravity. Also check for condensation on the frame. That too could be being absorbed into the belt.
These are just suggestions, i havent got a clue what it might be. But its things that pop into my novice brain.
I had a patch of rust at the bottom of the door pillar on the drivers side. Ive never found the source of the water that presumably caused it, but it hasnt come back since Ive been keeping the car dry at all times.