Upon meeting with Blackbeard and getting one of your new powers, you will unlock the ability to go after some treasure chests. The problem is that these chests are locked and require you to find three glyphs hidden in the area surrounding the chest. These glyphs are usually on the walls and glow pretty bright, so they should be easy to spot when you see them.
When you get to level 15, you can purchase a normal upgrade called the Collectible Finder. Doing so will place the locations of all of the glyphs on your map (same symbol that you have to find), making it a lot easier to find them. Note that while the glyphs appear under collectibles, the chests themselves appear under Targets.