At 23:59 the observer is seen walking down Ping Street as the Asian thug receives a call from the shopkeeper Astrid and Walter just spoke to about hookworms.
The glyphs for this episode spell out: HIDDEN.
Walter again sings the song "Van Amburgh's Menagerie" while waiting in the car toward the end of the episode.
Unlike other episodes where Walter cannot seem to remember Astrid's name he correctly remembers it several times in this episode. First when Peter learns that Walter was bitten, Walter asks Astrid to show Peter a picture of the hookworms. Later, expressing concern to Peter asking "Astrid was followed?" as he enters the car after being picked up by Peter in China Town. Then when he returns to the lab to check on Astrid he calls out "Astrid" looking for her to see if she's okay.
First time that Peter speaks Cantonese in this show. It is one of several languages that he is known to speak, along with: English, Arabic, Persian, Latin, Greek, Russian and Spanish.