Without spoiling myself: What do the secret vault symbols that you generally have to find in areas for their Badass ranks look like? I've looked around for a while but couldn't find anything. It would be less of a waste of time if I knew what I was looking for.
It is the same symbol that was all over Borderlands 1, and is still the 'saving' icon in Borderlands 2.
As you explorer them, you will find some small (like this screenshot), some large, some half obscured by objects, some on floors, some on walls, etc.
(screenshot from the video guide for the 5 symbols in Sanctuary)
This answer deals with finding an easy example in-game, and as such contains spoilers:
I have yet to find any others though.