It is not uncommon for the radiator to develop a coolant leak. A leaking radiator will require replacement. When a leaking radiator is replaced the entire cooling system should be inspected and if any additional leaks are found they should be repaired also.
Car Problem Reports
BMW 528i Coolant Leak From Radiator
BMW 528i Problem
Average Mileage: 82,666 mi (58,000 mi - 130,000 mi)