Hint: one of the notes you pickup provides a subtle combination for the safe in Rothwild office, but it makes no mention of safe at all and the topic of the note seems completely unrelated.
Explicit spoiler: code for me was 512. The note was an "accident report" picked up inside the slaughterhouse somewhere outside Rothwild's office. It mentions something like how many days or weeks it's been since the last accident, how many people got hurt, etc. The three numbers given in this note, in the order it is given, is the safe combination. Only played through once so not sure if the safe combination changes or not, but it should be whatever numbers are in this accident report note.