Shrike

A cold breath touched him: a faint change in air pressure that told him of an outside door being opened and closed four floors below. He moved quickly to a window and looked down. A forked blob of body heat was moving through the shadows of the courtyard toward the checkpoint at the gate. Grike measured its height and stride against the data he had gathered during his time as bodyguard,
and recognized Dr. Zero.

Infernal Devices, Chapter 17, the Chapel, pg 102/237