Kindly answer this question. (Question of Trust)

Solution:
Honour among thieves is a phrase suggesting trustworthiness within a group that is not considered trustworthy to outsiders.

In the story, A Question of Trust, the lady herself being a thief cheats Horace Danby which leaves him stupefied. He could not believe it happened to him and someone from his own profession did so. He believed, even thieves have some ethics when it comes to dealing with someone from their profession. 
The lady in red did not follow the ethics/ code of honour and made Horace land in jail.