The cipher note I agree is likely written by Brian before being interrupted- but unless the culprit allowed him time to do such a thing after taking him to the lounge he must have written it ahead of time. Perhaps the culprit put him on the couch and he shoved it there when they weren't looking?

I too am still confused by the book at the cutpaper note. The book being from the lounge could mean it was made after Brain was killed and they simply used what was available?