Flipper,
I agree it wasn't the typical smell of sliders melting, it was definitely the smell of plastic actually burning.
I inspected the seat, the muffler heat shield, underneat, etc very carefully (spent a long time going over every inch). There is nothing I could find that showed any signs of burning/melting at all. Even the bottom of the seat, where it was closest to the muffler and pipe area didn't show any signs of any heating, nor did the exterior surface smell at all. Only when I compressed the seat did the burnt plastic smell exit through the openings to the foam on the bottom.
I wonder if it's possible that some of the slide material was thrown up and stuck to the muffler (I didn't check the bottom of it too carefully) and was actually burning there?
I'll have another look at the bottom tonight.
I guess another remote possibility is something like a plastic bag got thrown up there somewhere.