At the beginning of Neverseen, Councillor Oralie gives Sophie a cache from Kenric - something that holds the darkest secrets of the Council that they want to forget.
Later, Edaline comes to visit the Black Swan and helps Sophie hide the cache in the void she can only access by saying specific words (221B Baker Street) with her voice. Sophie chooses this because when she moved up to the "older kids school" when she lived in the forbidden cities, her parents gave her this code phrase when she wanted to get picked up but didn't want the other kids to know she had called her parents. Her "Dad" was a Sherlock Holms fan, so he had chosen those words because she will never forget them.
During Cognate exercise with Fitz, Fitz accidentally says the code aloud, leading him to know that that is one of the ways to receive the Cache. Worried he might have messed things up, Sophie assures him that it only works with her voice.
At the end of the book, Keefe uses her voice when saying the code to give the Cache to the Neverseen. He later explains that he knew this code because he heard her Cognate exercises with Fitz. Keefe is not a Polygot but his Mother (Lady Giesla) is. He learns to Mimic from her.