You unlock this achievement when you discover the 15th secret area. You must discover all 15 areas in the same playthrough. Discoveries do not carry over. There are 73 areas in the game. Some are more difficult than others. There are more than 15 areas that you will come across in normal game play so the achievement is not missable
The following is a list of all 73 secret areas. They are listed in the same sequence as the walkthrough. https://tinyurl.com/Thief-Secret-Area-List
Your path through the game may be different. If you are not using the walkthrough, you can still consult it if you need assistance locating a secret area. The numbering corresponds. For example, if you need information pertaining to secret area 7/73 in the above list, search for Secret Area 7 of 73 in the walkthrough.
Unlike most of the game's stats, secret areas are not recorded. You are informed when you discover a secret area by the sound of a distinctive chime. If the chime does not sound, you are not credited with the area. When you hear the chime, make a manual save immediately after the sound finishes. You with discovering the secret area for the playthrough you are on. Even if you backtrack, for example by loading an earlier checkpoint save, the manual save ensures you get the credit for discovering the secret area.