Shadow of the Colossus: all secrets, hidden modes and easter eggs

There's quite a lot hidden around the world of Shadow of the Colossus.

Shadow of the Colossus (PS4): All Bonus Unlockables

Shadow of the Colossus Hidden Coins (PS4) - Shadow of the Colossus and ICO Guide - IGN

Shadow of the Colossus - Secrets, Unlockables and Easter Eggs ~ Games On Review

The last secret of Shadow of the Colossus. The most obscure secrets in video games

Has anyone already discovered this? Is it a secret room? Maybe the bluepoint easter egg? : r/ShadowoftheColossus

Shadow of the Colossus Easter Eggs (PS4) - Shadow of the Colossus and ICO Guide - IGN

Shadow of the Colossus (PS4): How To Find The Secret Last Guardian Easter Egg Cave - Gameranx

SotC - Very Hard Mode and 8 Hidden Colossus

Shadow of the Colossus players and their search for the secrets within - Polygon