Hidden locations are darted around the levels and offer little side missions. For example during the first mission the commentator will tell you that you just passed a hidden location unless of course you found it. They are mostly very easy to spot and since you only need 7 you can miss a few. If you look at the stats of each mission they will tell you how many hidden locations there are for that level. If you don't unlock this during your first playthrough just keep an eye out next playthrough. Look for trails that don't lead anywhere near the trophy animal.