Each zone in Dreaming City has a hidden magic chest. To open these chests, you need 3 arc charges, which should be quite close to the chest. Take the 3 bow charges and bring them back to the chest and they will be unlocked.
Below you will find a video guide for each of the chests and arc charges locations, have fun!
You can find more guides to Forsaken right here, on our Destiny 2 overview page.
How to open all Arch Charge / Orb Chest in Dreaming City
The chest in Rheasilvia is on a platform near the tunnel leading to the Divalian Mists.
The magic chest in Divalian Mists can be found at the giant door, from the landing place, head towards the huge door and look at the cliffs