Jeremy was with the police long enough,outside,for them to get a grasp of whether he was anxious at the thought of someone possibly still being alive,,or agitated at the time spent waiting. If you're experienced enough,you can immediately get an overall picture of that person.
Nothing like this was obvious,,except for the police to say he was calm. He would be calm because he knew nothing and was waiting for news from inside the farmhouse. How can you react to nothing ?