How does Raphael know that the police do not know that there was a key among the items inside the leather bag for which they are searching?

Raphael feels confident that the police do not know about the key, or at least that they do not know that the key belongs to a locker at the train station. He believes that if they had known, then when the boys arrived at the train station, they would have seen that every locker within the train station had been pried open.