Selma Payne's POV:
Benson found a homeless woman named Julie. After some investigation, he was convinced that Julie had worked part-time as a servant in the manor for a short period of time. However, the strange thing was that no investigation had mentioned her; not then, not now.
Moreover, it seemed everyone had forgotten that such a woman had once worked in the manor. Not to mention Julie's former colleagues, even she herself had completely forgotten that period.
All the people who had been investigated had forgotten about Julie, so no investigation involved her.
This was obviously strange. Julie was a lunatic, and the manor obviously would not hire a lunatic as a servant, so Julie must have gone crazy after she left her job.
However, her records in the manor had also been completely erased. If not for the fact that she was still wearing the servant's uniform, even Benson would not have been able to find any clues.