I have recently had to source a non obvious coolant leak on my facelift 3.0d. It turned out to be a tiny crack in the plastic return pipe on the neck of the second bleed screw. so it would only leak under pressure. I have sourced a replacement pipe but I managed to stop the leak by using a hose clamp on the neck of the bleed screw which seems to be doing the job until I can get time to change the pipe