1998 Nissan Altima Radiator is losing fluid

I am actually writing for my son. His Altima is losing radiator fluid almost daily. After parking his car at night the following morning it is common for him to find a large puddle and an empty radiator. I've looked at the car and I cannot find any evidence of where it may be leaking from. I've had a pressure guage put on it only to find that it is holding pressure properly and the radiator does not have any leaks. The hoses are fine and the clamps are tight. I've changed the cap but the problem persists. I am at a loss on this one and need some help. If you know of this problem and a solution to it I would sure appreciate a reply. Thanks in advance.
Bill