Unknown: Failed Mission
It is unlocked after finding all of the GPS Caches. After picking up the very last one of them, a chime familiar from finding a secret in the classic Tomb Raider Games is heard, and Lara says "A-ha! There's a note inside." The document can be read via the inventory.
I was sent here to investigate a new threat to Trinity's interests on this island. When I arrived, I discovered Mathias and his Solarii Cult. I believed then that Mathias could never acquire the Star Phenomenon, but I was wrong. I should have killed him when I had a chance, but now it's too late.
I've been compromised and mortally wounded, my mission incomplete. My time is short.
If you've deciphered the numbers and found this stash, then you must be my replacemet. Kill Mathias! He's close to reawakening the enemy that has long lain dormant. This cannot be allowed to happen.