My warning light (2010 X5 Diesel) came on at 2400 miles. The dealer said the res. was down by 12 liters and that they could find no indication of leakage. He concluded they had not filled it fully at the distribution center. They also had another customer a fews ago; same model, same urea deficiency and same diagnosis.
I expect to learn over the next few weeks whether I have a leak or not.
By the way, when the mechanic spills urea all over the engine compartment, as mine did, you feel like you're driving an outhouse for a few days until the odor burns off. Really pleasant.
Will keep you posted.