thats a wierd one if gasket was blown and you tested it while engine was running it would of shown some sign of pressure loss within 30 min ,,gee thats a weird one ..
 
the only other thing i could think of if the pressure is holding is either some broken fins in the water pump or rusted out fins  that doesnt allow it to pump water thru the engine fully ..
 
certainly is a weird one
 
i think i have 2 new spare head gaskets laying around if you want to chuck them on might even have the manifold gasket as well ... there not the real good brand name ones but if you want to give it a go let me know and ill express them to you
 
cheers

"the only other thing i could think of if the pressure is holding is either some broken fins in the water pump or rusted out fins  that doesnt allow it to pump water thru the engine fully"
 
That's exactly what I am tippin', or restricted coolant flow through radiator. But as BeeVee mentioned may now be subsequent damage also. With these sort of issues usually there is a sequence of problems not just one.

Just to illustrate how cool these V6 engines run, I have driven countless kms without Fan Blades, no Thermo fan, no Shroud and no Panel between rad Sup Panel and Top rad tank.............................
Thanks to a TOTALLY CLEANED OUT Rad, 74 degree thermostat, very clean Block/water-jacket, 50% AFAB coolant, and a well tuned engine.
Only disadvantage: without a fan you cant risk heavy slow traffic esp on hot days.
Main advantage: more power, less wear/tear, and better fuel economy. 
Am soon to try eliminating Fan Pulley/Bearing with shorter Belt.
Any cons; none
Any pros; same as above 3 lines up

So drove to work this morning after topping up the radiator again. I only use concentrated coolant and demineralised water. Park it up and a couple of drips on the concrete. Let it cool for 10 mins and opened the cap with towel of cause. No fluid loss. Then revved it up and fluid goes down slightly and back up when left to idle with no real bubbles to be seen. Watched for five minutes and put cap back on. Couple of dribbles start again. Then I turned it off and the dribbles or drops then turn into a constant flow out the overflow pipe but only happens when you turn car off. Pissing me off at the moment.

Based on what I have read there is no way you need to pull the heads off and you should avoid that like the plague.  Every skim changes how the inlet manifold lines up and seals because of the V shape and soon you get to the point where front and rear manifold seals fail.  
 
My guess is that the little raised ring that the cap rubber seats on is corroded and letting water past - it happened to mine.  You don't need to worry about priming with the heater on, the heater controls only move an air flap, there isn't a valve in the water system like on modern cars.  Slow bubbles could just be some cavitation in the pump.  If you had anything more sinister you would have a stream on bubbles and the pressure test wouldn't hold.  These things are hopeless in traffic on hot days so if it is corrosion on the cap seat get a new radiator custom made with more cores and fit a recovery system, there's a good reason why every modern car has one (A recovery cap without the bottle does nothing different to a normal cap), and while the radiator is out you might as well fit a new pump to save you doing it all again later.