In chapter 8 of Avi's The Man Who Was Poe, we learn a white button and piece of string were left behind in the bank vault after the bank robbery.
In this chapter, Edgar Allan Poe visits Providence Bank to see the vault from which the gold was stolen. He poses as Mr. Grey, a "private investigator from the Lowell Insurance Company" (75). Mr. Peterson, one of the clerks at the bank, leads Poe to the vault. Before Poe enters the Vault, Mr. Peterson says he was already sent to examine the vault and only found a "white button" (76). As Poe looks at the button in Mr. Peterson's hand, he sees it matches the same pearl button from the shoe of Edmund's sister that Edmund found earlier in the room of the tenement building just across from Edmund's own room.
Though Mr. Peterson claims the button was the only thing to be found in the vault, when Poe kneels down to examine the floor of the vault, partially because he feel like fainting, he finds a "bit of string" that he pockets (77). He also notices the air shaft in the vault is too narrow for a grown adult to enter, but a child could easily fit into the shaft. He later compares the piece of string to the "bell ropes of the Unitarian Church" and concludes that the string is not really a string but a "piece of strong rope" (155). It's the piece of rope and the button that help him reach the conclusion that Sis was kidnapped by Mr. Rachett so he could use her to steal the gold in the vault by sending her down the air shaft tied to a rope.