A man like Larsan, or Ballmeyer, could have had made duplicate keys.

As to the documents, I think Larsan had not intended to steal them, at first.

Closely watching Mademoiselle with the purpose of preventing her marriage with Monsieur Robert Darzac, he one day followed her and Monsieur into the Grands Magasins de la Louvre.

There he got possession of the reticule which she lost, or left behind.

In that reticule was a key with a brass head.