These lines are from T. S. Eliot's famous poem, "Macavity: The Mystery Cat." They perfectly encapsulate the central characteristic of Macavity: his elusive and baffling nature as a master criminal.
The first line suggests that Macavity maintains a veneer of normalcy. He can be seen in public places like any ordinary cat, "in a by-street" or "in the square." This makes his criminal activities all the more mysterious, as he blends into society seamlessly.
The second line reveals his true nature. Despite his apparent public presence, he possesses an uncanny ability to vanish from a crime scene. The moment a crime is officially "discovered" and the authorities arrive, "Macavity's not there." This highlights his cunning, his perfect timing, and his seemingly supernatural power to defy the law, earning him the title "the Napoleon of Crime."