Noticed this while looking around. This chest is there one instance and the next is gone. It appears to be outside the instance but the second you go near it you enter Seaside Sanctuary and the chest disappears.
So going near it makes it impossible to get. It only shows up on the minimap because I was able to click it before it disappeared but didn't start the minigame.
![7AOsMAv.jpg](http://i.imgur.com/7AOsMAv.jpg)