Missing person true story or "fake" true story involving artefact like ring or key. Read in 1990s.

[Pete]

As a teen in the 90s my school librarian gave me a book to read. I will assume it was YA but could have been adult too. It was about a missing person or persons and seem to revolve around an artefact like a ring or key or a even a pot or something. Maybe it found while camping but unsure. It was either a true story or was written as a fake true story perhaps. I remember at end of book was a phone number for police if you ever saw this person/s. I just remember reading it and it had a very haunting scary feel to it and was written factually like an investigation or non-fiction but maybe not written as a typical story.