Are you talking about an oil leak at the valve cover gasket or a water leak? It's not clear the way your first sentence is worded. I would assume that it is an oil leak at the gasket, which could be just a deteriorated gasket, lack of sufficient torque on cover bolts or deformation of the cover. To figure out if indeed there is communication of combustion gasses between the cylinders and the water jacket, just start the engine and with the heat exchanger cap removed pour water in till it is totally full to the brim. Continue running the engine and watch for air bubbles, all the while pouring more water in as it spills out. There may be air trapped in the system if it was very low before you start this so if there are a lot of air bubbles at first, just ignore that but after 5 minutes or so if bubbles continue to show as you run with the heat exchanger tank at the overflow state then there is combustion gasses getting into the water jacket either through a failed gasket or cracked head.
Be sure that you examine the coolant for contamination from the raw water side before you start looking for bubbles. I don't know what type of heat exchanger you have but they can have internal failures from corrosion and cause you to loose coolant while sitting. You can pressure test the exchanger easily to check its integrity.
