BMW Coolant level warning light?

I would think it's a faulty coolant level sensor at the expansion tank.

I however not heard too many problem with it. Not saying it can't be faulty, just not a common broken item.

First verify your coolant point and replenish if mandatory. If the point is right, then carry on with the previous answerer's suggestion. The coolant point sensor is below the upward thrust tank this is the black plastic field related to the radiator.