Every episode of Fringe contains a hidden clue that foreshadows something in the next episode. In Fringe #403 "Alone In The World", Walter has a book on his shelf titled "Astral Projectiond and Other Psychic Phenomenon".
In the Fringe episode "Subject 9", Cameron James was a cortexiphan subject who developed the ability to Astral Project:
OLIVIA: I thought you said you'd never seen anything like this?[Transcript for "Subject 9" from Fringepedia.net]
WALTER: Exactly like this, no, but something similar. 25 years ago, when Belly and I did the Cortexiphan trials, there was a boy who displayed the beginnings of an ability for astral projection.
OLIVIA: Astral projection?
WALTER: The ability to travel outside the physical body.
Here is a close-up view of the books: