My CDT has always done this unfortunately, know of a least 2 others that smell of burnt diesel fumes when sitting in traffic with the windows slightly open.
I know another that doesn't smell at all
, Is there a stain on the bumper around the exhaust? I am sure the problem is down to exhaust design, the exhaust fumes come out the exhuast bounce off the floor and get trapped behind the bumper, air flow then sends them up around the sides of the car. I noticed when I had a problem with smoke earlier this year I was able to see the flow around the car when sitting still (cough), I intend to modify something when I require a new exhaust.