The cap on the expansion tank allows water to leave if pressure is too high. It has happened to me before when I had my oil changed at a quicky lube, the guy filled it too full and when I drove home it was overflowing and leaking out of the expansion cap. I thought I was screwed, but it turns out that it likes to be on the lower part of the tank. So if your was too full that could be why. I would reccomend getting a callibrated temp gauge, the stock ones are not nearly accurate enough.

84 760 ti 360k km