Smoke coming from vents in expedition with everyting off.

Your ac blower doors that direct the air to the vents are vaccum controlled check your vaccum lines going to the ac most of the time it is a leak near the vaccum check valve. The reason it is only top vents is because it is the default setting in the system

You mean from the aircon vent.Your cooling unit need a good chemical wash.Ice is forming in the cooling unit, the air outside is hot thus is the cold air forming smoke from contacting the hot air.

You have a leak in your heate core, located behind the dash on the passenger side. Common problem, I have the same vehicle and had the same problem. Parts not that expensive, but many hours of labor. Dash has to be removed from vehicle!

You can usually remove the old vents using a small flat bladed screwdriver and inserting it in the space between the dash and the vent (where the vent pivots) and gently prying up until the vent comes out. Replacing the vent is easier, just very carefully snap it back in the pivot holes. If the space seems a little tight, use the screwdriver to gently pry the dash out alittle till the vent goes into place.

Blend door actuator broken or disconnected.

If it is a kind of sweet smell then your heater core needs to be replaced. It's leaking and you should keep an eye on your coolant level until you get it fixed. The smoke is coolant. Hope this helps.

You are going to need a new heater core, here is how you replace it, go to the link for video of the whole job
http://www.youtube.com/watch?v=qa7TiLRn9oE

Are you sure it's smoke? Probably not smoke, the coils are freezing up and the air is so cold it condenses and looks like smoke coming out.

Hope I was avle to help, glad you up and going.

mode door actuator , it's a electrically controlled motor under the dash on the heater / A/C unit . Videos on youtube showing how to replace them .