Secret Locations

[Regularity]

I've seen a few passing mentions of hidden locations in various discussion threads, but it's going to be pretty easy to never find out about them if you just so happened to not look in that thread. So I think we should get a thread specifically for hidden locations to consolidate them all in one place.

The two I've seen mentioned so far:

The False Eye item. A copy of the red barrier key behind a holographic wall in the Central Cavern, available after your first game reset. Allows you to bypass the normal requirements and enter the red barriers right away.
https://steamcommunity.com/app/1839430/discussions/0/592895445665116724/#c592895445665126197

Secret message for quest. Destructible wall with secret message for Gilli-Vell (tank lady with giant beak helmet), giving you a different path in her quest chain.
https://steamcommunity.com/sharedfiles/filedetails/?id=3470018239

If you find any others, post them here.