In the novel Postern of Fate, Miss Collodon is a woman employed by Tommy Beresford as a researcher. She prided herself on being able to research anything. When Tommy absent-mindedly muttered a few lines from Flecker's Gates of Damascus to himself, Miss Collodon thought he wanted him on a question of poetry. Tommy used Miss Collodon to research names such as Mary Jordan from Somerset House. Later, knowing that there was a census at the time when Mary Jordan was staying with the Parkinsons, he asked her to get a census list of who was staying in the house at that time.
Miss Collodon is described as "thin, emaciated, had grey hair which was slowly passing through the stage of recovering from a peroxide rinse designated to make her look younger (which it had not done)." On the colour of hair, she is trying "various shades of artistic grey, cloudy smoke, steel blue and other interesting shades suitable for a lady between sixty and sixty-five, devoted to the pursuit of research."
That Miss Collodon is present at the dinner hosted by Mr Robinson at the end of the story might suggest that she is more than what she seems to be, and that she might have some connection with the intelligence-security world.