The ice warning goes off at 3 C as the temperature drops, and it usually takes a few minutes driving for the sensor to trigger that, from whatever temperature the garage was at. If the temperature you are seeing is the exterior temperature, 3 C, then that is the beeping.

I am in Vancouver, and it started several days ago, for the first time this year. Then it snowed today....

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2007 X3 3.0si, 6 MT, Premium, White

Retired:
2008 535i, 6 MT, M Sport, Premium, Space Grey
2003 X5 3.0 Steptronic, Premium, Titanium Silver
2002 325xi 5 MT, Steel Grey
2004 Z4 3.0 Premium, Sport, SMG, Maldives Blue