Cold case
From Tardis Wiki, the free Doctor Who reference
The disappearance of Pyrovillia was considered a cold case by the Shadow Proclamation by the year 2009, having occurred over two thousand years prior. (TV: The Stolen Earth) The Judoon also treated the case of tracking down the Doctor to be a cold case, which they finally resolved when they arrested their thirteenth incarnation. (TV: Fugitive of the Judoon, The Timeless Children)
