Hot, rubber smell from engine vent

[GrantG36]

I have recently purchased a 2 year old Smart ForFour. It's a very low mileage car at only 5,100 miles on purchase. Since then I have done a couple of lengthy journeys, and a reasonable amount of commuting. I have, occasionally, experienced a hot, rubber tainted smell coming from the engine vent in the rear nearside quarter of the car. Oil and water levels OK. Temp gauge registering in the normal range. Should I be worried?

[Thrumbleux]

I'd open up the engine hatch and have a look when it happens and try and pinpoint the source. Possibly/probably just road **** thrown up onto exhaust but just in case there is something more serious that could cause a fire, I'd check for source.

[ColSmart]

I have had a plastic bag kicked up under the car and covering the catalytic convertor in the past and that gave off a burning smell for ages, had to scrape as much plastic off as possible.