(This is my first post on the FFG forums.)
I haven't seen anything about this, so I figured some might be interested.
I just got Bones of the Yopasi, and contrary to what I thought, it turns out that it's not a promo for Arkham Horror, but --- like the previous one that came with The Lies of Solace --- one for Elder Sign.
It's an adventure card called The Log of Persephone.
Here's what the advertisement at the back of the book says:
"Sunk beneath the ocean waves near the town of Kingsport, the Persephone's dark secrets were thought to be lost… until now. The Log of Persephone has been uncovered from its watery resting place, and is on display in a new exhibit at an Arkham museum! Take advantage of this special mail-in promotion to unlock the mysteries of the ship's disappearance with The Log of Persephone, a new Adventure card for Elder Sign."