Borderlands 2 has numerous hidden secrets littered across the map. One type are hidden Vault Symbols in every area. There are usually 2 or more in each location.Finding them increases your "Badass Rank" which gives you points that you can spend to strengthen your character a set percentage in things like critical hit damage, grenade damage, gun accuracy, shield recharge rate, etc. Sanctuary has 5 hidden vault symbols, 3 of which can be difficult to find. Here's a quick vid showing their locations.
How did I not know about these!? Thanks for the post mate!
Probably for the same reason I didn't know there was a Lord Of The Rings side quest after 2 years of playing the game. There's just a massive amount of things to discover in this game.