I have a hyundai getz and i have to put a pint of water in the radiator every day.

ive taken it to the garage, they kept it for 2 days, did a pressure test, and they cannot find a problem, they say the waterpump is ok, the head gasket is good, rad is good.....im really puzzled, does anyone else know of this problem.??????????????

engine does not overheat (unless i forget to fill it up with water!)