could be crap from the road.
But to be sure check this at night after a nice long or high speed drive. if the converters or the area around them are glowing orange you may have a blocked converter causing a heat build up and some smoking.
They should have a nice coat of rust on them at this point, no black crap or anything wet. Also check from above with a flashlight to see if anything is dripping on the exhaust from above or any wiring touching it