Did the FBI agent *want* Hank to think he was lying about the serial numbers?
The agent notes that a liar will often cover his mouth when talking. Mere seconds later, he speaks of recorded serial numbers, and briefly covers his mouth. No actor would touch their face during a tight shot unless directed to do so.
Perhaps the filmmakers wanted to tell us that the agent was lying, and that Hank fell for it. It seems unlikely to me, however, that a trained pro would legitimately commit the same gaffe he had *just* talked about. Was this some kind of psychological trick the agent was playing?