[BURN-BROS]

Glenn,
The water could be going into the combustion chamber. There have been reports of a hydrolock condition that breaks the top piston ring. This breakage gets above the cylinder "sleeve", then forces said "sleeve” towards the crank resulting in catastrophic engine failure. Typically you can detect combustion gasses in the coolant at most radiator shops. Hope this is not your condition...

Another scenario could be your system likes to be low due to higher pressures in the coolant system. Does the system loose all of its coolant in the reservoir over a long period of time or always a pint low? Possible fix is higher pressure cap if it applies to a 996. To verify, get the car up to operating temps and look for coolant to dump out. A steady stream of air bubbles in the reservoir, if in fact you can see the reservoir, would be a condition of a blown head gasket and the first condition I mentioned.

I am not up on my 996 engines so you will have to look around for yet another opinion. Good luck.