Salvation's Edge Secret Chest | Between 2nd & 3rd Encounter | Orange Room | Destiny 2: Final Shape 

This hangs around for a while after others open it, so you don't necessarily have to wait for everyone else before opening it.
This chest is tucked away in the back left corner of the large Orange room AFTER the outside jumping puzzle with the Taken Boop Blights. It is a long way after the Herald of Finality fight!
The chest will only give you loot which you have already earned in the Raid. In my case, I got another copy of the SMG I just got!