I once had a tiny, hidden leak on my 2001 3.0i.

It was at the front end of the heater pipe (hard plastic, not hose) that runs along the left (driver's) side of the head / upper engine block.

It was so slow, and being hidden under a bunch of other stuff, and with a long way to drip and evaporate before it reached a visible area, it took a long time to find it.

Part number: 11-53-1-705-210

In my case, it was a new part I had put in there, which had defective o-rings. Replaced with Genuine BMW, as I should have used from the start.

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2001 X5 3.0i, 203k miles, AT, owned since 2014