Perfect crime
From Memory Alpha, the free Star Trek reference
A perfect crime is one that leaves no trace nor evidence despite a thorough investigation.
In 2369, Ilon Tandro argued that failure to punish Trill symbionts for crimes committed in past lives would create a perfect crime. In his hypothetical situation, a symbiont could kill someone, switch hosts, and get away unscathed. (DS9: "Dax")
