The Achievement called
Super Secret Stash (Opened Zarpedon's Chest)
The Mission to open the Chest is located in Pitys Fall and called "
The Secret Chamber" and is available after defeating "Bosun" during the Story Mission "Intelligences Of The Artificial Persuasion" (Chapter 5). You have to find 3 Echos to open the Door to Zarpedons Chest. Follow the Video for the Locations. (sorry my english is to bad to explain as Text where the Locations are)