Hi,
I also lost a lot of coolant after i had renewed the chains and waterpump, somethimes its very hard to see where its coming from.
i would advice to feel with your hands down the hoses, and waterpump and see and feel also smell if its coolant that is leaking.
When you have drived the car for 30 mins or something, turn it of and let it cool down after 1 hour go get a look under the car if you see it dripping coolant anywhere.
Also read this topic.
http://www.astraownersclub.com/vb/showthread.php/555444-z20let-loosing-coolant-with-no-leaks
I also readed on a Dutch opel forum, about the z20let that, it somethimes can happen that the coolanttank got some little cracks on the attachment points and it only leaks when it is on boost and that its very hard to see.
Also inspect the radiotor plugs, a plug of my radiator also leaked, the rubber ring whas completely gone and needed a replacement.
After that the problem whas stil there and i noticed that the pipe that goes to the waterpump whas leaking at the O-ring but i dont know if the turbo engine is the same as a 2.2
After all somethimes its so hard to see, specially when it doesnt leak very hard or meaby the coolant just vapor away while driving or it gets burnt do the heat of the engine. so be patience
The mechanic said to me when it is your headgasket it runs like crap and got a oil smell in the coolant tank. or coolant in the oil.
You can test if there is coolant in your oil, get your dipstick out of the engine and put a lighter on the oil, if it makes popping noise its coolant. i dont know if this is very true i just heard this tip
Justmy2cents