Mystery Substance on Muffler Pipe

Plastic garbage bag melted you ran over it blew under the car whatever. That's all external A leaking exhaust pipe wouldn't cause that You would just have black soot that looks like melted black plastic trash bag or a lightweight plastic container whatever it'll just burn off.

 

Pretty much any debris that melts that you see on a road while driving could be the cause of that... I think @Tombukt2 probably has it right, though from now on when I do lots of highway miles my mind will likely think of this post and what it could of been... Way more fun than slug bug or the license plate game.

 

The wind from your forward motion 55 60 mph will try to push that bag right around the pipe and then the heat will cut it just like a hot knife which is a wire that's hot you cut foam with like butter so the bag whips under the car the air still is pushing back on the bag it pulls it right across the pipe like it's cutting it whatever pieces of the bag that stretch and do whatever will wrap around the pipe somewhat and sit there and burn off until it gets hard as a rock and then you have what you have left You can scrape that off with a scraper for what I don't know maybe you'd like to be under the car You shouldn't smell it after oh five six eight minutes it'll burn up to pretty much nothing but hard nothingness You burn everything out with that exhaust pipe heat there's nothing left to melt burn or what have you.

 

Plastic burns off eventually. Or at least it can. I ran over a white plastic grocery bag once in a Mazda and the car smelled like melting candle wax for weeks. After that the exhaust pipe was clean again.