I’ve been on the Whitney main trail about 15 times, and have occasionally lost the trail near Mirror Lake. But the terrain is pretty mellow there, so I’m not understanding how a long fall is likely. I’m sure that there some drop-offs a few hundred feet away, but it seems like they would be easy to avoid, even in the dark.
This aerial might help explain the problem in this area above Mirror Lake. The cliff has a chute that someone might attempt. Combine this with the more distant views and a map to complete the picture.