Completed the DLC 100%. The thing that took me the longest time was opening the safe between father Riccis office and Cloaka Maxima. It's very rusty and is sitting on some kind of wooden shelf. It had the last artifact/item for me to collect. Searched in the vicinity for the combination and went through every note in the DLC with no luck. Eventually just brute forced it and figured out that the code was 5742. Still didn't find any hint leading to the combination though.
The safe contained an artifact, a small handgun, a knife and a golden mithra mask. I suspect that these items belonged to Ricci.
