Smoke in vehicle cabin in frigid sub-zero temps?

I just moved to Fargo,ND from CA. I have a 2001 ford expedition v8 fuel injected, 5.4l. i have only been here a few days, i have an astra start on the vehicle and i start it every few hours dues to the temps being below zero most of the time. the first morning that i have been here i turned on the heat defrost and i noticed fog/smoke coming from the vents. it doesnt smell like exhast or burning electrical. i opened the hood and did not notice any smoke leaking from any tubes. It only happens when i first start the vehicle during the day. when the vehicle has been warmed up for some time it does not happen. only in the morning at the first start. I checked the anti freeze and it was way below the line, i filled it back up and am waiting to see what happens. this has never happened during the wole life of the vehicle until i move to ND. any suggestions on what it could be?