In the back room you'll find an Infusion sitting on a desk. Drop down on it and head inside this little maintenance area. When riding the skyrail for the first time you'll see a platform with one guy standing on it. Use the key on the chest in the bedroom and you'll find the bottle inside. Head back to the Lansdowne Residence after getting the key from the Order of the Raven. The bottle is up on the stage table next to the key to the chest in Lansdowne Residence.
In the Order of the Raven head to the large room with the stage.
