As far as I can remember, coolant expansion tank, thermostat and hoses (and water pump too if you are so inclined) are almost a regular maintenance item, i know when I bought my car the PO said he replaced everything but the water pump at 60k miles.

Cooling systems on BMWs from what I gather on here can be finicky, hope it was just a gremlin for now but maybe you are in for some preventative cooling maintenance in the future!

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2018 Ram 2500 6.7L Cummins 68RFE
19k miles -Bright White/Black - Big Horn Sport - Crew Cab Short Bed
2013 X5 35D (CEO's) - Born on 5/17/2013 -
82k miles - Alpine White/Cinnamon Brown/Premium Pkg, Sport Activity/Premium Pkg and Sound/20" Style 214/Running Boards