How do you know it leaks? Fluid level drops or by puddle on ground? If puddle it may be from the overflow. May be overheating even if temp reads normal. Radiator cap may be weak and letting the coolant come out the over flow. Replacing cap "may" fix it. Answer 1 is not incorrect but, I will respond to the part of the question about running "cool" in spite of the leak. IF the temperature sensing and guaging system are working properly, then the "normal" reading is correct. If the leak is small enough, and ENOUGH coolant REMAINS in the cooling system, then that would allow for adequate cooling, and thus a normal reading. Also, if the leak is small, and by regular "topping off," you keep the cooling system FULL, then one would expect normal cooling, and thus a normal guage indication. j3h.