Got the coolant level low message for the first time myself a month ago and it just so happens it was pretty cold that day. I had the car in for maintenance on that day and they just topped off the coolant. No errors since. Just top it off and you might not have an issue anymore...

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2008 550i Sport
Space Gray, Black Dakota, Bamboo Anthracite
SAT, Sat, Shades, Cold Wthr, Comf Access, Nav,L7, PDC, LUX V3 AEs, BMW Perf. Grilles, HID fogs, ACS Roof Spoiler, M5 Lip Spoiler, RPI GT Exhaust

Others:
2006 Corvette Conv, Z51,6sp (wife)
2007 Honda CBR1000RR
2005 Triumph Daytona 650 (wife)