the.com/cold case

means An unsolved crime, usually serious like a murder or disappearance, that has gone without new leads for so long that active investigation has stopped — though the file stays open.

from A straightforward policing metaphor from the mid-20th century: a criminal 'case' that has gone 'cold' the way a trail goes cold — the scent, momentum, and urgency all fading until there's nothing warm left to follow. 'Cold' had long been used for stale trails and leads in detective slang before 'cold case' solidified as the standard term, later popularized further by TV shows and dedicated cold-case police units.