it's in the cave complex, where the marker is
If I remember correctly, you just have to walk past the wall there and there's a chance your character notices it. It may be an invisible perception check, but try going past there a few times and you should get the dialogue that opens it up! I remember unlocking it by accident before the story with Dulrig, Rueben, and the Emerine Crystal, but if it still doesn't work then try progressing story a bit more and going back again.