i had a problem like this with my 89 Camaro Iroc-z i guess at some point i hit a bird and it got stuck up in one of the intake vents for the cold air. I finally couldnt take it anymore and looked way up in there and to my surprise there was a rotting bird with maggots all over it wedged up in the intake vent ugh it was gross.

melted plastic bags on exhaust are very common...especially on California freeways where it seems that the garbage trucks that cruise at 65 mph to get to the dump site tend to lose half their load!

My other car, known as 'the ant pusher' has grabbed plenty of debris on my commutes...driving a lowered car can sometimes turn small pieces of debris in to obstacle courses on the freeway. I've even hit something that was disguised in between lane reflectors that hit my tranny pan so hard, it cracked my torque converter lock-up solenoid and made the rest of my drive home quite interesting.

But yea..any time you smell something burning, it is most likely something melted to your exhaust.