If the car has been sitting it probably has a bit of condensation in the exhaust. As the exhaust heats up it causes it to steam off, sometimes it can take a while to completely dissapate. Evenings are cooler, so when the warm, moist exhaust hits the cooler ambient air it condenses into a fog. We've had some really damp weather here lately (Seattle, Kentucky
) and it seems as though some cars never quit steaming.
To be certain there are a couple items to check: 1) Watch for coolant loss in the reservoir. Remember it will fluctuate up and down as the engne heats up and cools down, so it's best to check it first thing in the morning, before starting. 2) You can go to a garage and have the cooling system checked for carbon monoxide content, if positive it would indicate combustion gasses in the cooling system.