But think about it, Prater would believe that eventually the BHB would infiltrate the group at some point to prey on them. Im sure initially they thought Red would be joining the group. But I also think Prater knew that the tell would be when the group members suddenly start disappearing. And again I also think Prater is aware Dexter is not Red. Dexter Morgan doesnt exactly have a low profile. Hes been in the news as a victim impacted by the trinity killer. Hes got a mugshot from being arrested. Its very possible that Prater not only knows he isnt Red, but knows his real identity is Dexter Morgan. And from there, knowing that he used to work with Miami Metro, alongside the suspected BHB? It becomes so easy to connect the dots, for someone like Prater.