Thanks for the great detail on the system operation. Yes, the float bobs up correctly when the cap is opened. Since the float comes assembled with the expansion tank, I assume it was built properly with its magnets. Likewise, I bought the new electronic sensor from a reputable dealer so I think all of these expansion-tank related components are new and working. I even tried putting the old sensor back and it didn't change anything. I am back with the new sensor and the message persists. It's driving me crazy. The system seems to be operating fine, level steady at full for weeks, so driving the car is not a problem. But that message is always there and chimes in when you start the car and when you shut if off. It also leads to complacency and could mask a real leak if ignored, which I'm doing. Maybe there is a software bit stuck in memory somewhere. This is such a simple thing in cars - "detect low level of a fluid" - and yet BMW makes a Rube Goldberg system to do it. Ridiculous.

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2006 X5 4.8is
LeMans Blue over black
Style 168 rims
Blackout grills and hood vents
Aux jack