Why is there a small amount of smoke coming out from under my hood when I am idling at a light?

I just noticed in the past few days after driving on a highway and then exiting and driving on local streets that when I idle by a traffic light there is a small amount of smoke coming out from under the hood. We just had the oil changed a week ago, could that have anything to do with it, meaning did they forget to do something that would cause this?
I can lift up the hood and look underneath when the car is shut off and it is not smoking or hot at all...Any ideas?