Just before the climax of the movie, Randy Cartoff, an ex-convict who tapes a live confession, admits to hurting and killing Margot before he kills himself. David is devastated.
Preparing for Margot's funeral, David comes across an ad from MemorialOne, a funeral casting service. The ad features the same person who appears in the profile picture of the YouCast user fish_n_chips. Earlier in David's investigations, he learns that Margot has become incredibly friendly with this girl.
Bubbling with suspicion, David uses a Google image search to track down the stock model used in both of these images, who has no clue about Margot's investigation. While trying to contact Detective Rosemary Vick about his findings, he learns from a colleague that the detective volunteered to take on Margot's case from the very beginning. Very suspicious indeed.
David looks up Rosemary's profiles online, coming across an earlier article about her police outreach efforts, in which the main image features her with Cartoff, the purported killer. David relays his suspicions to other detectives, who then arrest the detective at Margot's vigil.