I recently had a water leak that didn't really do much until the engine got hot (which obviously pressurises the cooling system). Again no gunk in the oil and no bubbles in expansion tank. (3.2 V6 GT)
However if I left the expansion tank cap off overnight then I found a small puddle of water in the morning. I then did a diy pressure test on the cooling system and found the leak straight away. I had bought a new cap as originally I wasn't sure whether the pressure valve had failed and I was losing water as steam when hot. So for the diy pressure test I used the old cap, removed the rubber washer, cut out the valve with a dremel/rotary tool then got the valve from an old bike inner tube and cut the rubber to size, drilled a hole in the centre of the cap and super glued the valve in place. I then replaced the rubber seal and glued to the rubber of the inner tube for good measure. All you do then is screw the cap on as normal and use a bicycle pump to pressurise the system. Don't go mad with the pump though, you don't want to over pressurise the system and cause more problems, I think about 20 psi is correct but don't quote me on that. Maybe someone else can answer that for me, anyone??
Anyway, my leak was traced to failed/perished o rings on the metal bit that the radiator hoses and heater hoses connect to where it connects to the engine block. I presume the 3.0 V6 is similar to the 3.2!?
Hope some of this helps!