I suspect Nagi just was forbidden to interact with her after the incident (I mean Chihiro is lucky that her parents didn't push the issue with the police or insurance or anything like that), and by the time she was able to escape her parents' leash, she was a teenager and not able to locate Chihiro easily. I bet she randomly came across something written by her (or maybe she intently searched for something written by her as a way to find her), and then was able to get more information by bribing publishers (this is where the stalking comes in).