Not sure how many players explore everything possible. But I am finding more and more areas that exist outside of the confines of the actual map (HUD or Menu Map). I am a huge TES fan, and all the years playing those games I have learned to try to jump/climb everything and look everywhere. There are tons of loot containers and even major loot chests stashed all over the place in BL3.
Also if you see redish brown cables follow them until you find either a power sources or one that looks like electric flowing
You will have to use the environment to creat connections for secret rooms or powering down shields