2002 330Ci Same deal. Developed small leak in expansion tank. Took me a while to find where it was leaking from. Removed coolant tank and it appears that when the original tank was made there was a very small crack in the plastic that just got worse over time. Had I not found it earlier it would have been a disaster. No lights came on to warn me. Just that familiar sweet smell of antifreeze every time I would get out of the car after driving. Bought a Meile replacement from eBay for pretty cheap. Did it (pretty easy replacement job) at 70K and still working good 5K later. I did not replace the cap as the rubber inside was still very good. I did drain flush and fill with new coolant mix, checking to make sure the coolant level remained towards the top as it comes out the top. Checked it a couple other times over the next month. Added a small amount of fluid each time. Have to get the air bubbles out of the system. Just goes to show: A well cared for car goes a long way.