Unknown usually refers to a track by an unknown artist ([unknown]), which will not be scrobbled onto Last.FM. It can also refer to a placeholder artist for untitled tracks for album pre-releases on Apple Music.
These are photos from concerts where Unknown played, so they may be of other bands that played the concert as well.
Unknown Mar 12, 2015 Brunswick East, Victoria, Australia Uploaded by Andy J Ryan
Dear & Headlights / Kinch / Rajiv Patel / Unknown Sep 24, 2009 Cambridge, Massachusetts, United States Uploaded by Treyj1234
unknown Aug 20, 2006 Cambridge, England, United Kingdom Uploaded by Craig Hatfield
Unknown Sep 15, 1978 Kansas City, Missouri, United States Uploaded by Scott P