Suddenly a woman ("The Girl," played by Amanda Seyfried) appears in the movie. Her data block appears as "Error - Not Found" when Sal passes her on the street. That's not supposed to happen. It turns out that The Girl is able to access people's visual record, and erase selected portions of that record. This of course is great for criminals who want to erase the evidence of their crimes. It's also great for regular people who just want to hide things that they're embarrassed about. Anon charges a fee for this service, she also erases herself from the person's memory, and everything seems to be going fine. Until we discover that people who are hiring her are also getting killed. We can't see who is killing them, because of course that is erased from their record, too. Since The Girl is off-grid (illegally) and has these unheard-of-before abilities, she is suspect number one.
Sal meets with other detectives and the department's technology experts in a big concrete room to figure out what to do. There's no way to track her down because, by definition, she's off-grid and invisible to their technology. The group decides that Sal should go undercover.
…
During the end sequence, we learn that Anon hid herself by storing bits of herself in everyone else's public record. As long as she had her algorithms to put everything back together, she was able to hang onto her memories, but no one could find her because the bits were scattered among millions of records.
https://en.wikipedia.org/wiki/Anon_(film)