Are you sure its actually disappearing. There is a big misconception about antifreeze level to many people. You're radiator is not supposed to be full when cold when most people check it. Antifreeze expands when it gets hot, so it goes into the overflow. If your overflow tank has a hole in it, or your line from the radiator to the tank has a small hole in it, if its still a sealed system, it can't draw the coolant back into the radiator since its done by vacuum. So I'd say if its staying within an 1 1/2 to 2 inches to the top of the radiator cap, that is normal. So fill it up. Take it for a normal drive getting it good and hot to normal temp for several miles. Let it cool and check your level. Then call that your normal point, and check it regularly for awhile. If its staying at that same level, you're not losing any.