was it raining and cold? if it was and your lights were on its pretty normal, unless there is loads of steam.
Also was you following another car?, maybe exhaust from the car infront?
These things have caught me out in the past, i thought i had a cooling problem and it worked out it was just exhaust or water on the lights.
if above dosnt apply, is the car overheating / an or loosing coolent?
if yes try to find out were from, look for tell tale marks of drying antifreeze (it will be a whiteish, or the colour of your coolent, and crystalized, you will see marks maybe on surrounding equipement if its a pinhole. try to check the rad as these have been known to go.