To unlock this achievement, you have to find all the secret stashes in Moira Asylum, the creepiest level in the game, which is considerably less creepy when you visit it in Thief 3: Deadly Shadows. There are 5 stashes, and the description doesn't specify what kind of stash they are. Those collectibles you get for your collection? Apparently not.

You have to collect 5 golden pocket portraits, which sadly aren't anything special compared to other loot and not worth much anyway, only a measly 10 Gold. You'll earn this achievement though! Here is a picture of the pocket portrait, from stash #4, and a collectible next to it. There are several silver pocket portraits in the asylum, but these don't count.

The first stash will be in the Men's Ward. On the first hallway on the right as you enter, there will be a cupboard. Climb on it, climb the narrow platform above it, and find the portrait on a vent opening.

The second stash is in the same hallway, further down. From the last stash, jump to the narrow platform on the other side and go further. You'll see this pocket portrait on a vent to the right.

Still in the Men's Ward, go around the corner to this room. Open it, crawl through the hole in the wall in the next room, and find the pocket portrait above the door. In the same room behind the turned bed is the Women's Ward Key, which you need to progress. Don't forget to grab it!

The next stash is in the Women's Ward. When you enter, on your left will be a broken elevator shaft. Use a rope arrow, go down, and then turn around to face the elevator. There will be pipes on a wall in front of you, and 2 golden items. Grab the pocket portrait, and next to it is the collectible Rose Brooch from the Flowers Eternal collectible set. Grab it as well to progress your collection achievement.

From the last stash, climb the stairs to the floor above. On your left will be a room which you can either unlock with the Women's Ward Key, or go through some broken bars. On the corner of that room are some bookshelves, and a vent on the wall. Open it, and turn around to the left. The final pocket portrait will be sitting there on the ground.

Credits to PowerPyx for providing the video for the location of the stashes. I know that there are numerous video guides about this achievement already made, but I made this guide for people who prefer to look at images instead of watching videos while hunting collectibles. Just in case you prefer a video, you can watch theirs here: