Obtaining the Note
If the party chooses to revisit Ayuthay during the Grave Eclipse, they will be informed that Ku-Tsung and Ku-Embra have escaped the prison. Going to the ancient tree where the Tree Flute was used, the corpses of the generals will be seen, and using Spirit Sense on either corpse will trigger a conversation about a map which they hid in Kaocho.
The note reads as follows:
This shows a pirate treasure's location. Coming from the south, go west when you reach a reef shaped like a hand.