I have '01 330xi and '01 530i. My 530i had a small coolant leak where I would find a small puddle of coolant on my garage floor. I too had a hard time finding the leak. I did a pressure test. Got to 15 psi and it would very slowly lose pressure. I determined that it was the thermostat because of small traces of coolant I thought I saw. I replaced the thermostat and water pump. Next day, same puddle. Turned out it was leaking from the plastic side of the radiator on the passenger side. I replaced the radiator and upper/lower hoses. My coolant pipes under the intake were changed about 20k miles ago and my expansion tank just 2 years or 2k miles (not my daily driver) ago so left those alone. I'm glad I replaced all those coolant system parts and the mechanic that did my reservoir used green fluid so i was glad to get that out of there too. At 240k miles and no history, there is no harm IMO of replacing the cooling system. If it fixes it, great. If not, hunt continues but at least you're good for another 60k miles on your coolant system.