[Batman]

If there are no associated problems why would you say it is a head gasket? I would check under the water pump, the coolant lines, the radiator, radiator cap, radiator hoses the cap to the coolant reservoir etc etc, all of which are more likely then a head gasket. A slow leak could go completely unnoticed unless you really dig in and look for it, especially under the water pump. Once you eliminate all that easy shit then start worrying about a head gasket.