Windows(not the windshield)fogging up in my 07 Focus..why?

I do not see any anti freeze leakage,carpets dry,do not smell antifreeze yet every time I drive this thing the windows fog badly.Is my heater core going? The heater and defrosters work fine.Everyone I have asked is baffled by this as am I. I have tried running with ac on as well,no help.Cracking a window only helps marginally.Frustrated!!Calling all mechanics HELP