The glyphs in this episode spell "mirror."
While it is hard to spot the observer in this episode he does appear at 24:16 standing just behind the police car on the far right you can make out his silhouette featuring a bald head, black suit, and a black briefcase.
Just before re-creating Olivia's car accident, Walter calls Astrid by her actual first name.
The farm house visible in this episode is the same as used in X Files Home (season 4 episode 2),
When shapeshifter Charlie is typing messages to the other side the mirror shows the reply being typed out. Only the keys on the far side of the typewriter are shown moving which would make the first reply of "unacceptable" be impossible.