Biography
The artist name "Unknown Composer" maybe often be the result of a mistagged song. However, a compilation/soundtrack album may literally credit the name "Unknown Composer" if they can't find the name of the artist at the time of release.
The most prominent example of this (by the number of plays on Last.fm) is a track called "Fat Man Part II," which appears on the album "Dramatic Funk Themes Vol. 3 - Roaring Rare Grooves Action & Detective Breaks 1972-1983," a compilation of library tracks from said era.
Biography text from Last.fm, available under CC BY-SA.