The axle nut always has a cotter pin, clevis pin, locking ring or some other such device. Loosening or removing this nut is a two-step process and can only be done intentionally...and I suspect the dealer at the 75K service did something with this wheel and didn't reinstall the nut properly. (You should have somebody check the others, too.) If you don't already have one, get a detailed list (without telling them why you want it) of the work they performed. You need to ascertain whether they have documented a procedure that would have required the removal of the axle nut (i.e., to check bearings, CV joints, abs sensors, etc.). You'll then know whether you have clear recourse to collecting damages from the dealership. I'll bet you do.
--
(98 S70 T5SE misc mods, mostly lighting, red calipers) (92 940GLE)