Several possibilities, Bad head gasket, try doing a compression test see if you have 1-2 cylinders with low compression.
Leaking heater core, you would probably see /smell something out of the defroster vents.
Any puddles under the car?, could be leaking frost plugs, cracked block, bad hose somewhere, bad rad. Try a pressure test on the cooling system and see if the pressure drops off.
Basically a process of elimination.
the best way to check a leak is to use a pressure tester. pump it up to 15 psi. and watch and listen for the leak. but beware, if the leak happens to be a head gasket or intake gasket this test could finish it off. also any other weak points in the cooling system may be revealed
I once had such a leak. Was losing coolant, but never saw any drippings or other signs until one day I stopped at a service station and raised the hood. I immediately saw steam rising from the intake manifold. Sloooooow leak in heater hose was dripping onto intake/exhaust manifold and evaporating before hitting the ground. Check it out while engine is hot. Slow leaks never reveal themselves while the engine is cold. With the engine up to full heat, raise hood to look for steam. It's free and quick. Definitely worth a shot.
how often do you drive your car? when i didn't drive mine for say 3 days, it started leaking from the connection of to the hose, because the hose contracted from the cold, and wasn't as snug. so what i did was go around and tighten all the hose clamps ( they all needed tightening)