It's funny that a firefighter is worried about a little smoke.
I reckon JustinL is right on this. I expect you've a slight oil weep around the cam belt cover area. It doesn't show on your drive because the under tray catches it. When you're moving at a decent rate of knots you get the symptoms you describe.
Have a look in the rear view next time this happens - see if there's a slight haze behind you that disappears when you slow a little. That's what happened to me until I had some new seals put in during a belt change.