What you need to do is pressurize the system with a coolant pressure tester, and look for the leak. I recommend removing the entire cabin filter assembly and the engine cover. You need to get the car on a lift and look from below, and also look from the top. To look from the top, you really need to climb up on top of the car and get your head as far back as possible. Arm yourself with an inspection mirror and bright light. Feel around all the hoses until you feel something wet.

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2014 BMW 328i Xdrive
2011 BMW 335i M-Sport
2008 BMW X5 4.8i Sport
2000 BMW 528i 5sp