I live in CT where we are having one of the coldest winters on record. My average temps within a week of the low level indication were in the teens & twenties. The urea solution freezes at 23 deg. F. The passive tank is not heated & will freeze. The active tank, pump & devilery tubes are heated on the active tank. So I know my passive tank was frozen from time to time. The dealer said the tank was in fact empty altogether. The info on the system does say that more frequent refills of the active tank will be required in cold weather. No reason is given but it is readily recognizable that this would be due to the passive tank being frozen, thus unable to allow for a pump transfer to refill the active tank. All that being said, I'm still not sure what happened. I'm waiting for BMW to respond as to whether the cars are shipped with the tanks full.
2009 X5 35D Black Sapphire Tobacco Nevada