Originally Posted by chubbs

I'm not discounting any of the theories in this thread, but I've had a wet seatbelt once (2016 car) and I'm pretty sure that I'd shut the door on a small part of the belt, which hadn't retracted properly. It rained quite hard, the belt got wet and the water wicked through a large area of the belt. Of course, in a lot of cases the wet belt will retract as soon as you open the door, so you won't know it's been trapped.

Ever since then I've been careful to make sure the seatbelt doesn't get trapped in the door and the problem hasn't recurred.

I am NOT saying there couldn't be another issue - just letting you know what happened to me.