Any signs of a leak? i would be checking hoses/raditors for any visable leaks, also when you park up overnight slip some cardboard under your car then check it the next morning for signs of a leak and this will give you a general area to check.
if you don't have any leaks in the engine bay then the only two places i can think of would be -
A) Heater matrix (car steaming up recently/soggy carpets)
B) It's going into your oil (i.e head gasket issue)
Start with checking for a leak and go from there