Early in the movie, David skims through Rosemary's Facebook page. The detective's cover photo features a seemingly harmless Agatha Christie quote about a mother doing everything that she can for her child. Well, it turns out Detective Vick would really do anything for her son — even cover up potential involuntary manslaughter.
The fact that Rosemary seems like such a do-gooder should arouse suspicion in the avid thriller fan. (You don't need to look further than Adora in Sharp Objects or Rose in Get Out.) When David first searches the detective, he sees a bunch of articles about her accolades and community service projects. In one of the articles that he clicks on, the detective is posing with ex-cons for coverage of a police outreach project. The main person that she poses with is none other than the man who, in a live video, falsely confesses to killing Margot before taking his own life.