What do Miles and the FBI find in New York when they approach the warehouse Miles suspects is used by the killer? What impact does this have on Harper?

Miles and the FBI finally track down and enter the warehouse in New York that Miles believes the killer has been using. When they enter, they find that the killer is not there. They enter with explosions, but Berkmann just keeps typing. This is when Harper realizes that the killer must know they have been watching his typing error rate, and that he must be close to Harper's house.