Board Thread:Spoilers!/@comment-6175354-20151120142327/@comment-25926288-20151120142912

It's clearly a Darkness-EMF meter. I was totally kidding, but it would explain why Rumple thought Emma was coming, with Belle hiding (not like they sit there all day like that) and instead it was Hook.

It probably would be a bit of a spoiler isf they said it tells if the Darkness is near and then here pops up Hook, lol.