There have been some threads on 2016 X3 xdrive28i rear axle seal repair costs. I recently had my tires replaced and the technician noticed leaking from both rear axle seals and advised I get them repaired by a BMW dealer. I never saw a drop of any fluid in my garage, so it must have been a seep of some sort. Interestingly, I had the vehicle in for an inspection and oil change 2 weeks earlier at the BMW dealer and they didn't notice this. I brought the vehicle in for another inspection, with the heads-up this time that the leak had been observed, and they confirmed. Repair for both seals ran $1680 total and took about a half a day. Just passing along some data for those that might run into a similar situation.