My advice from personal experience with two 540i radiators, is to check around the plastic "neck" area on top right (when you're standing in front of the car facing radiator) of the radiator. That is a common problem with fine hair-line cracks, which you wouldn't even notice normally. Also, on the 2000 540's, the upper assembly hoses for the radiator, where it goes into the engine, they need to be replaced. It's in the service bulletin. If you check them out and see greenish deposits around the joints, that's one of the possible causes of a leak. The updated replacement is a simpler setup and has less joints.
2012 Porsche 911 GTS
2008 ford F150 Foose Edition