dissturbbed

I have a 1998 3.8 L mustang and had this mystery leak for about 6 months, leaks about half a cup of coolant every day under driving conditions. So far I have done a head job "twice". Had both heads pressure tested, shaved, the whole nine yards. Changed radiator, water pump, hoses, and still have this leak. When i pulled the heads i looked everywhere for a cracked block and could not find any cracks. I also plugged up the heater core. There is no sign of a puddle, the radiator isn't bubbling, freeze plugs are dry, and there isn't any sign of a leak anywhere besides the reservoir loosing coolant. Any suggestions? My next step is too pressure up the coolant system and remove both exhaust manifolds and see if there is coolant leaking. Could it be a cracked lower intake manifold?