The most common thing is the plastic thermostat housing starts to crack and cause this you would need to put a pressure tester on the system so you can watch for where the coolant is leaking out.
I just got cut-off replying to you. Is this a simple process? Like I said before I can only see the leak from under the engins and behind the wter pump. Can you send me via email and exploded view of this area in question?
Removal and Installation