Do you know where the coolant went?

One possible issue is the heater control valve. It can get stuck, and then the system won't respond to changes in requests for hot coolant. If it shows any signs of leaking, that would make me more confident that it was the cause of the problem.

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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