For some time I have been topping up coolant (litre per month say)decided to look at it more closely today.
Pressure test for short duration showed no leakand there are no visible leakage just some corrosion /chalking around the thermosat housing.
However I have noticed some nicks on the rubber expansion tank seal inside the cap. I have cleaned the metal external sealing ring and refilled the engine with water. But need to get new cap seal next week.
Could this cause the leak as I am dreading a head gasket repair!

 

I have a 2000 2.7 that had coolant leaks too. I changed the expansion tank (cracked), auxillary water pump leaked, coolant lines to the back of the heads leaked and a line that went in between the heads. Leaks everywhere. Good Luck.