Uncommon for diff to brick at 60k miles?

So I have the car back in my possession and the shop director is looking into the car's history at the shop. We discussed in particular my concern with them having to refill the diff back when they did the Inspection II they did before putting it on the lot, along with a few other minor things they were telling me they needed to do which I think are related to work they did as well in prepping for delivery. All of that, when he looks though and sees it was related, will be covered under their warranty so I'll not have to pay for it.

It all just gives me the impression that they're sloppy, and that's probably the worst reputation a shop can IMHO. Shady I can deal with-you know what to expect with the work. But sloppy, it's like having to nanny the people you're paying money to fix stuff, not knowing which thing isn't going to be done right. It's frustrating.

However, I had him ride along with me and he pointed out what they were hearing. It's a slight noise, kinda like a whistle, and I have been hearing it for months. It's just so faint that I didn't think anything of it, figured it was part of the car's sound. I'm just kicking myself now that I didn't take it in two months ago and have it replaced for free.

So, we'll see if they replace it under good-faith to the warranty or what happens; like I said, he's looking into the car's maint history. I'm hoping that they'll agree that a diff devoid of fluid should have been investigated and offer to fix it, but, we'll see.

BMW diff oil is listed as lifetime, which none of us believe. But it takes some expensive oil by BMW. You might get them by saying they didn't fill with correct bmw diff oil.
Good luck.