When the P.D got to investigate a location of a suspect it seems that the culprit was expecting them to to show up.

Season 2 Episode 10 "Shouldn't Have Been Alone"
The Intelligence Unit investigates a house with rigged traps and learn of a connection to a professor's sphere and a mentally ill student of his. They investigate several places rigged with traps and learn of a darker motive of the student. Elsewhere, Burgess recovers with Platt and Ruzek at her side.