There's only one way to find a cooling system leak that's not easily visible and that's by pressurizing the system with a special and easy to use tool that can be rented at Auto Zone or other such places. You pay $75 + tax, but get it all back when done, so you can figure out what's wrong for free, then take it back. This way you don't get jacked around. If it's simple enough, just fix it yourself and save the time and grief of dealing with the garage.