Call of Cthulhu
49 Achievements
12-15h
GOG
The Secret Cave
Discover the mansion’s secret passage
56.5%
How to unlock the The Secret Cave achievement in Call of Cthulhu - Definitive Guide
TheWelshHunt
TrueAchievements
TrueAchievements
Story related and cannot be missed.
This is for completing chapter 3, getting behind the bookcase by either:-
Prying it open with a crowbar OR
Figuring out the globe puzzle to have it unlock automatically. (For getting all of the achievements though, use the globe)
2 Achievements unlock!
Here is my 100% guide for this game and to see where and how to unlock this, go to 36:30 in playthrough 1 video.
Hope this helps
Playthrough 1
Playthrough 2
This is for completing chapter 3, getting behind the bookcase by either:-
Prying it open with a crowbar OR
Figuring out the globe puzzle to have it unlock automatically. (For getting all of the achievements though, use the globe)
2 Achievements unlock!
Here is my 100% guide for this game and to see where and how to unlock this, go to 36:30 in playthrough 1 video.
Hope this helps
Playthrough 1
Playthrough 2
Easty
Story related and cannot be missed.
After chasing the thief at the end of Chapter 3, you’ll be required to open a secret passage. There are 2 ways to open it, but one of them will also net you A brilliant mind 
so make sure to refer to it.