"Has that tattoo grown since last you looked?"
The Sigil-Ridden Navigator - that poor soul - made a deadly mistake on the last ship he worked on, which indirectly cost him his memories.
Quite some time ago, the Navigator and his brother, Richard, joined a ship's expedition to the East. The ship's captain was heading towards Kingeater's Castle, and then he planned, due to lack of supplies, to go farther east in search of land. Eventually they were forced to return, but the Navigator had misremembered their course, and they sailed northwest instead of west. As their supplies dwindled, the crew resorted to cannibalism, the Navigator among them. His brother Richard, along with other crew members, was killed and sent to the pot to feed the others. Once they returned to London, they swore to never speak of what they did. One of them broke this vow, and the Sigil-Ridden Navigator fled (to avoid the hangman's noose) to the Avid Horizon to plead for forgiveness. The Admiralty chose not to answer.
Overwhelmed with guilt, the Navigator built a small rowboat and traveled to the nearby Chapel of Lights. He had a Correspondence sigil engraved on him that would erase his memories, allowing him to forget what he did.