The noun mystery is a common noun, a word for any mystery of any kind.A proper noun is the name of a person, place, thing, or a title; for example:Mr. Mystery, mystery-fiction series by James Lee (Jim Aitchison)Mystery Island, Vanuatu (Oceania)Mystery Spot, Santa Cruz, CA (see link below)"Manhattan Murder Mystery", 1993 movie with Woody Allen, Diane Keaton