To dive deeper into the mystery and the real-life events that inspired it, check out these sources:

The episode ends with the contractors discovering a hidden, Prohibition-era tunnel in the basement. Dean and Nora find evidence that someone has been living there, and the episode closes with them chasing a mysterious figure through the passage.

Ellie discovers a Facebook group for former students of an English teacher named Roger Kaplan , who used to have his students write "love letters" to houses. This makes Roger a primary suspect for the origins of the Watcher's poetic and creepy writing style. Fact vs. Fiction