There are lots of places to inspect. Check the easy stuff first. A slow leak out of the thermostat housing or upper radiator hose can evaporate almost as fast as it leaks causing you to lose coolant at the same time making it hard to detect. Also check the the the small hoses behind the engine that go into the fire wall and all their connections.
Checking the heater core is good advice. But also..

Look for coolant drip around the the transmission bell housing. This could mean a bad water pump o-ring in the back of the pump. Also the water pump itself can have a slow leak from the shaft seals or housing seals that will evaporate before it will drip out in some cases. Also the tube that goes to the back of the pump can corrode and cause problems, again look for the drip on the back of the engine...Good luck.