Missing Note

[Mudbill]

I know I had some trouble finding this one myself. Are you sure you're looking in the right location? IIRC it's in kind of a corner because there's a fence next to the desk. Have you compared the location with a video to confirm that it isn't where it should be?
Have you checked if it maybe fell down somewhere?

If it really isn't there, I suggest you try verifying the game cache via Steam, and then load a save file from the level before where you find it. Then try and see if it appears.