I would get the sensor checked at the dealer, if it doesn't match the dipstick indication.

I wouldn't worry about it too much, as the i-Drive warning should come on when it is down 1 litre, and there are still 7+ litres of oil in the engine. The add oil warning has been interpreted as the engine being out of oil by some, but in reality it is just a reminder to top it up. Mine doesn't take any more than one litre when the light comes on.

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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