I have an interesting issue. I have a 99 VW Passat with the 1.8 Turbo. Had the low fluid light come on, filled it up and heard water running out the back of the motor. I could see the water running out, but it stopped when I pushed in a sensor. Some google-fu later I found out I was missing a clip. New clip and o-ring later I was feeling pretty proud of myself, when the warning light came back on. Checking, sure enough the bottle was almost empty and water was coming out of the same spot. There were a few shards of the clip left that fell out when I reached in to feel if it was still there.

What would cause that type of pressure in the coolant system? I don't think it is a head gasket issue due to a lack of beautiful white smoke, and no loss in power.

I replaced the clip (again) and had tried babying the car (under 2k RPM) and it has worked for over a week, this clip is broken, but the shards are holding the sensor in place... so my water has stayed full, but the cold season is rapidly approaching, and I have to put the unholy G12 back into it before it freezes.

Any ideas?

Shouldn't the radiator cap (or equivalent) be bleeding off pressure before that point? What's the level in the overflow tank look like?

Jesse could be right on then, might be having the high pressure due to the radiator cap not venting right, and the high temps could be a water pump issue, as far as where the coolant is going.... Put a big piece of cardboard under it and let it sit, if its not leaking then let it idle and check or put it nuetral, rev it and check

Try replacing the pressure cap first.  It is the cheapest of the possible causes.