The conditions required for it to appear are currently unknown. They could have allways been there, it could be a Event, or perhaps the player needs to achieve a certain level or complete a certain quest. The chests are mentioned in this article, which dates back to 2012.
The chests reappear after some time, just like the plants do after you have removed them with the Botany Skill.
The green dots in the image below represent the locations of Unlocked Chests in Nightfall Citadel (4 locations currently):
When the Unlocked Chest is opened, you gain 1 Scroll of Enigma.
References to Unlocked ChestEdit
- Opening a Unlocked Chest is a requirement for opening Soul Power Gate III.