Bought the X3 in 2016 as a brand new car. Driven off the lot with 0 miles on it. In 2019 and before that as well, the vehicle will tend to lose a very small amount of coolant (enough to set off the "low coolant level" warning) over a 2-3 month period. Other cars we have owned never have lost any coolant. Taken it to the dealer 3 separate times, once they replaced the coolant expansion tank after I noticed residue around the lid, but the issue is still unresolved. Not very big of an issue because performance is unaffected and there always seems to be enough coolant in the system, it just needs more of it every 2-3 months.

Update from Dec 12, 2020: Update 12/12/2020:

After refilling the coolant in mid October, the car had lost that refilled amount just 3 weeks later. Took it to the dealer once again and they still could not replicate the issue. This is the fourth time that we have taken the car in here and it is absurd that this problem has not been resolved yet. Will be getting an oil sample next time to see if potential head gasket failure.