Seems it's true... no coolant level sensor. (Which makes no sense to me, even old 2000's BMW's have a level sensor.)
https://f48.bimmerpost.com/forums/sh....php?t=1736540
In either case. Something doesn't add up with your vehicle. I agree that SOME type of warning should've appeared. Coolant must've leaked onto the floor or into the oil. But if it was in the oil you would've smelled it from the exhaust.
Call your local BMW dealership and ask them if they're willing to perform an inspection on your vehicle to diagnose what happened.