96 Cavalier, unfounded coolant leak

My daughter has a 96 Chevy Cavalier with a 2.2 liter, auto trans with about 180,000 miles on it. The problem we are having is that the coolant level keeps going down, and there are NO visible leaks, no coolant in the oil, no nothing. Been thinking on a headgasket leaking into the combustion chamber, but then wouldnt there be coolant in the oil? On a 50 mile trip it will go down 1-2 quarts. Cracked head another possibility? Any thoughts would be greatly appreciated. Thank you!