On this page of our Call of the Sea game guide, we list the locations of all the story logs to be found in the Prologue - A Family Heirloom.
Chapter - Prologue.
Location - in the suitcase lying on the desk.
To get the key you need to open the suitcase.
See the Suitcase code page for a description of the puzzle solution.
Pick up the photo on the left. It depicts the main character's husband. You'll find the key on the back of the photo. It is a key element of the game's plot. Once you pick it up, you can leave the cabin.