I just unlocked this one after having a lot of trouble with it. I completed the chapters in order of Dolores, Rakan, and Lucy, but I made a save in the bathroom, right at the end of Lucy's chapter. After completing the last chapter, I went and picked up the Indifference tape, played it to completion, and went up to check the sink. If it wasn't there, I reloaded my save. After a couple of tries, the oddly sized key was floating above the sink. I've seen others post that they completed the chapters in a different order and got the key, so try with a save at the end of whatever your last chapter is.