That incident really messed with Mark and Walter's heads real bad, especially Mark. And here I was thinking Walter was the one that got in an incident. I'm guessing it has something to do with a cliff and a fall. Maybe the necklace was something given to forget rather than remember. Oh, they're a compass, nevermind then, still feel like it made them forget tho, for their sake.
Whatever it is, we can only wait and see.
(Kinda wish there was a spoiler tag here like in disqus so I can comment more spoiler-y stuff.)