You didn't say which engine you have, I'm going to guess the 3.7 V6. Just some general info for others reading. If it was the water pump most likely the main return hose would show some collapse and the reservior tank wouldn't be as empty. There is a main heater return line that comes out the back of the engine that when it leaks sometimes does it on a route that puts it onto the exhaust which burns it away and you don't see the leak underneath. If it is that multi-junction fitting at the front, definitely change the O-rings. I don't remember if that engine has a lower-intake with coolant passages, the gasket for that in some Ford V6 like to go bad around the passages and send the coolant into the combustion chamber burning it away, with some also going into the oll pan so check for increased level on the oil dipstick. My suggestion is change the O-rings and if more disappears, then get a pressure test done on the system for a better idea of a different leak.