I was playing on Expert and had to rely on the YouTube walkthrough attached to this page to find an extremely hidden (read "invisible") key, and there was also a well-hidden switch. Spoiler below if you don't wanna skim through the video:

***
***
***
***
***
***
***
***
***

KEY: In the room with the safe, keep pressing the Use button all over the mattress until you grab an invisible key that can open the safe. Yes, you can't see that key. It might as well be under the sheets. I couldn't find any clues about that key in any of the notes throughout the map.

SWITCH: In the same room, the switch you need to press is behind the desk.