Any funny smells in the cabin like maybe coolant? A pinhole leak in the heater core will drop the coolant level slowly and it only happens when hot and up to pressure.
A not so easy problem is a leaking head gasket.
1999 Jeep Grand Cherokee, Laredo (White) with 4.0 137,000 miles
2006 Chrysler 300C, 5.7 (Black) 98,000 miles
2010 Hyundai Santa Fe with 3.5 and AWD (Black) 65,000 miles