If you are losing coolant it could be a number of things- a small leak in a hose, water pump, intake, thermostat, head gasket, bad cap, cracked overflow tank. I know you said you pressure tested the system but did you do it correctly? You need to bring it up to 16lb and let it sit for some time. IMO if you had a bad cap, you would be seeing higher temps and visually coolant marks on the radiator neck or side from the overflow.
Also, you don't need to have water in the oil for a HG to be blown or leaking. You could have a hole or small tear around a water jacket and the coolant is getting burned off. Do you see any white smoke under acceleration or when idling. The best way to check is to rent from Autzone an exhaust gas detector and dye. The dye is blue in color and vacuum will pull the gas from the top of the radiator through the dye and up the tube. Pray to the auto God that the dye does not turn color to a green. That means you have exhaust gas in your coolant and a sure sign a HG is gone.