This is the most frustrating and bugged achivement in the game. Secret areas are hidden places that play a chime sound effect when entered. Best way to do it is to create a new save and go through them all, because you cannot tell wich ones you entered already and wich not. There also is no statistic or way of checking your progress on the achievement.
Here is video of all secret areas in order:
Keep in mind that this achievement is bugged. Every time you enter secret location and there is no chime in the backgorund
it is not considered as being found. From my expierience loading saves causes this so try to avoid it as much as you can. There are some fixes for when you don't hear the chime:
- If it's during any chapter try loading last save, if it doesn't help you got to restart the entire chapter (Press esc and choose "return to the city").
- If it's in the city you got to load last save and do any chapter and finish it to "reset the city" then try again.