From Longman Dictionary of Contemporary Englishsomebody’s trail/scent is coldsomebody’s trail/scent is coldTRAIL/SCENTLONG TIMEused to say that you cannot find someone because it has been too long since they passed or lived in a particular place I tracked the boy as far as the factory, but there his trail went cold. → cold
Examples from the Corpus
somebody’s trail/scent is cold• The trail seemed cold until a woman in Mississippi recognized Pearson's face in a wanted poster. 