A pressure tester tool can sometimes find the leak. They run about $50-$75 and can be found at auto parts stores, and sometimes rented from there. Attach it to the coolant reservoir filler neck, pump up the pressure, wait overnight, and check for any coolant leaks in the morning. Also check the pressure cap with the tester; they periodically wear out and need replacement. They tend to be pretty cheap.
I believe I have an external head gasket leak on my car, since I have noticed dried coolant where the head meets the block. There was no leak on the garage floor with the pressure tester.