In several different circumstances, most involving either a monster or a dead end in a mountain range\inland lake area.
I've stumbled over it three times now, just make a bunch of scouts and tell em to head on out. One of them will eventually drop out of Autoexplore, and just stand there. The lazy coward!
http://www.screencast.com/users/Mazzarin10/folders/Jing/media/b9a89f43-d355-44ef-8095-59dbaf806e1b
