1999 Ford F-150 smoke

Truck was exhausting a small amount of white smoke on first start up of day. This went on for about 2 weeks on and off. There was no smell other then common exhaust smell of a vehicle running a little rich. No rough idle. I checked the coolant level, it is right where it shold be. I checked for signs of moisture ie. Dipstick and oil cap, all OK Weather had been hot and humid. Truck runs smooth and quietly. I suspected a bad head gasket. Now the weird part. Everything is back to normal, no smoke in the morning anymore. Truck runs fine except still has a hint of rich exhaust.