Jürgen knew who Harris was all along
Jürgen's last task as a Stasi agent was to destroy or at least disrupt Section 15. He failed, but then, through his one of his few contacts, Nurse Erfurt, he was able to find Harris and use his predicament to destroy Section 15.His misquoting of the Stasi method of determining whether someone is lying or not was a deliberate test on his part to assess Harris' mental state.
