Good day!
I might be picking a little here, or maybe it's my old age, but "WTF" is okay... Spelling it out, not so much...

I would suspect you have an issue with your temp control valve by the firewall... this valve regulates the amount of hot water going thru the heater core... it might leak at wide open... IMHO, that seems to be one of the few logical reasons it would only leak at the cooler temps...

Good luck and let us know what you find!

Cheers

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2001 E53 X5 4.4i (03-07-2001 build date) White exterior / hellbeige interior (6MT swap vehicle - Manny)
2002 E53 X5 4.6is (12-20-2001 build date) Imola Red exterior / M-texture interior (Red)
2003 E53 X5 4.4i (06-26-2003 build date) Silver exterior / Black interior (Silvester II)
2008 Mini Cooper S ragtop (04-17-2008 build date) Dark Silver exterior / Gray interior ( Topless)
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