I’ve got nearly 500 hours into Advanced Story mode now and have known for some time about the hidden chests ISIC and flying classes can get to on The Algorithm, but didn’t know until I had time to experiment by myself, because there were no Story peeps to be found this morning…
So anyway, in the image below you can see the two hidden chests above the left area towards the back.
As Rath, without his Double-jump, what I did was jump on the console to the very right.
Then jump left to that central door frame.
From there you can jump to the medium height ledge to your left.
Then you can jump up to the chests!