I agree with MadeInJapan, but you might want to also look around the radiator.

I had some smoke that appeared and disappeared like that, especially when stopped at a light, and all I smelled was the normal engine smells. Finaly turned out to be pinhole radiator leak that was found more by seeing antifreeze coolant strain one day when under car for oil change.

From under the car done you see any indication of any fluid after the car sits overnight? Try putting a large piece of cardboard under the car after a days running about. Then analyze any drips - Black/brown - oil, greeninsh - antifreeze, reddish/brown - ATF