Any odors from the smoke?
I have had a couple of occurrences where a piece of trash blowing about on the street would attach to the hot catalytic converter or other exhaust component, and subsequently, melt or burn. Some would leave a residue that would have to be scraped off, while others burned off by itself.
2009 ML350 (84K) - Family vehicle
2001 CLK430 Cabriolet (71K) - Wife's car
2005 BMW 645CI (124K) - My daily driver
2012 Mustang V6 (60K) - Daughter's car