I've had two coolant problems with my Ford trucks and neither may be what's going on with yours, but you can't ever have too much info!
1) I had a slight leak at the EGR in my diesel F250 that leaked out in the passenger side wheel well. It wasn't enough to see it on the ground, but I could smell it.
2) My diesel F450 had a blown head gasket that didn't cause coolant to leave the system, but instead caused exhaust to enter the cooling system, and over-pressurize the cap. There was no loss of power while this was going on, but the Check engine light eventually lit which alerted me to it. The inside of the overflow was black with soot.
Both of my problems were fixed at no cost to me under warranty (thank goodness as the second was a whopper!)...