don't listen to this pin heads like Pimpreza.. my quess he has no idea about cars .. just like taking the piss out of people..
it seem like your mechanic did not bleed the cooling system the right way and if you have no leaks then it will take water out of your over flow bottle until or the air is out... best way to bleed it is to turn your heater taps on take the cap off and have the engine running at 2000 rpm until your themo fans kick in..
if you still have the problem after topping the over flow bottle a couple times.. i would be getting the mechanic to pressure test your cooling system to see if their are any leaks before you start doing some damage. i hope that help..