Our Secret Knock is a written note found in Death of the Outsider.
My servants tell me that you came by earlier when I was composing, and they refused to open the door. Apologies from the depths of my heart! I'd given dire threats that I was not to be disturbed for anyone. The admiring throngs who gather outside my home have become burdensome recently. Crying for autographs and interviews until I can barely hear myself think, much less create. But of course, I don't want to shut you out of my home with the masses.
From now on when you visit me, knock on my door six times, with a break on the third knock. My servants will know it's your personal signal and will have strict instructions to admit you right away. I hope you will come to me soon, my love.
This note can be found on a desk in Eolina Rey's apartment during the mission Follow the Ink.