This is what came in the mail:
It's from Biscotte & Cie, a yarn company in Quebec. I don't remember exactly how I found them. I've been wanting a self-striping yarn in a watermelon colorway for a long time. I even tried to dye some. It came out okay, but more like strawberry than watermelon. Anyway, I that saw Biscotte & Cie had some really great colors in self-striping yarns. I also saw that they had a mystery sock club so I signed up. The club yarn is the blue/green color. It also came with a cute heart shaped tape measure. Look, even the retractor button is a heart!
I've got the first clue. It's mysterious for sure because it's a small number of stitches knit back and forth for the leg.