In The Slipstream Con, when Kellen is injured, he sings a song about a seal lord and a human bride.  Tal and Vanya don’t recognize the song, but if they did, they’d be able to trace it back to Kellen’s homeworld- a bit of information Kellen hasn’t shared with anyone since he left it.

If you’re wondering where the inspiration for the song came from, take a listen to Heather Dale’s “The Maiden and the Selkie”.  (And pick up her album The Green Knight, which is fabulous.)