To be able to detect hidden things, you have to have at least two points in the Detect Hidden stat. You only get one point when you level up, so it'll take at least 2 levels to get the ability, unless your race starts with a point.
Once you've got the ability, hidden things will show up on your mini-map as treasure chest icons. As you approach them, your controller will vibrate and the item will glow. They're typically things like wells, piles of rocks or tree branches. Once they've been found, search them. You can take as many of the items as you want that are held within, but you have to at least search them for them to count toward the achievement.
These are scattered all around the world, so as you complete quests, you'll eventually come across 25.