So: this should be a story related. I'm writing this one just because it didn't unlock on my first run.
The idea is to check the hideout while on the "Finnland" and you're looking for the 4 missing pieces.
The vase's slice is inside a monument. Wait a little bit inside, and Finn should ask something about the mausoleum.
Let me know if you find any other way to unlock this one.