Took my wife's 2007 E70 X5 in for its first service on Friday. (Did have an oil change at 7500 miles) Technician found that the front differential was leaking. They have had a couple of cases where the differential was leaking and went undetected until the vehicle broke down on the highway with a locked up differential. Might want to take a quick look at yours to see if you have a problem.

I never had any oil on the garage floor yet the level was down significantly.