Suggest you crawl underneath and look at the hub where the axle fits into it...if you can see any of the reluctor ring, then it may have cracked and wiggled out of position. I just had this happen on my c280...BOTH rings had cracked and become loose enough to move inward on the axle shaft end to make the ABS sensor warning go off.

My guy said the rings arent the root problem, it's the axle that gets rusty and it expands enough the crack the ring, which on my car are stainless steel, and were in surprisingly good shape, even at 17 yrs old. So all they did was clean everything up, corrosion-wise, painted the axles in zinc paint and welded the rings back in place

Alternative was new axles, and ummm at 1300/pce...not gonna happen