My V6 also lost water intermittendly ... the water was the normal clear / green but the reservoir would be empty every now and again to my great dismay. Try as I may I could not find the leak. Untill I popped my head under the car to find the cause of a cluth pedal on the floor ... only to find a few conspicuous drops of green stuff hanging from the heater pipes under the driver's seat. Turned out to be a loose clamp. The reason I could not trace it is it only leaked when using the heater
Maybe this helps ...
Francois