A detective was usually someone who worked in the field of law enforcement, such as a police officer; whose job involved criminal investigation. Private investigators were also detectives. Both Sherlock Holmes and Batman were both considered the World's Greatest Detectives during their lifetimes.
Mrs. Cadwalader hired some detectives, three of which she described as the greatest "in the world," to help her with the security of her diamond that she wore in her pendant. Despite the presence of the three detectives, her precious stone was still stolen.