"Northwest Passage" was the original title of the seminal cult TV show Twin Peaks (1990). Like this episode, "Twin Peaks" was also about an FBI agent teaming up with a local sheriff in a small Pacific Northwest town to solve the murders of local young women. Also, like Twin Peaks, the small town is known for its delicious pies.
The glyphs in this episode spell out: RETURN
Walter identifies the alternate universe building from episode 'Jacksonville' as the Zelazny Building. Roger Zelazny was a fantasy and science fiction author who wrote the 'Chronicles of Amber' series in which the Princes of Amber can walk between alternate universes.
On Peter's Sat Nav it shows that he is in Snoqualmie, Washington. This is one of the filming locations of Twin Peaks to which this episode bears several similarities.
The Observer is standing behind the police tape with the crowd as Peter meets Sheriff Mathis.