I am not to concerned with a little cooland loss. the venting is normal on the modern car. All of my BMW's need a top off every year or 15000 miles with about two cups worth. They are twin turbos run real hot and vent extra pressure. My new jeep Grand Cherokee is doing the same and I can smell it venting in the garage. It took about a cup the first 12000 miles and there is no leak.
Now with that being said, when I drive my Cruze hard on a hot day and turn it off, I can hear the coolant boiling in the coolant feed to the turbo and the bubbles bubbling up to the cooland tank. This builds pressure and gets released to atmosphere (the water portion). This will always be the case on this car. If you make the seal stronger or install a higher pressure tank, it will crack. My fellow BMW friends don't complain this much about having to add coolant and maybe that is because the tank is black and not clear and holds way more than our Cruze. Now if the water pump is leaking, different story but GM has stated that seapage is normal with our style of pump.