The game contains a number of secret areas. They are always behind a 4x1 set of fake walls, in out-of-the-way locations (though tokens often lead up to them).

The problem with the secrets is, even if you missed just one, your only option is to... look harder, in another full playthrough of the game. That's why I thought it was important to have this list, which is best used after finding what you can by yourself, when you don't know which levels to look in. Look at your list of achievements and see what's missing.