1. A system for determining detection certainty in a cascade classifier, the system comprising: at least one computer system; and a cascade classifier application executable in the at least one computer system, the cascade classifier application comprising: a first sequence of one or more detection stages configured to determine whether an image is associated with a rare object, each detection stage configured under normal operation to reject the image when the image is determined to be unassociated with the rare object, and propagate the image to a next detection stage when the image is unrejected; a second sequence of two or more detection stages configured to determine whether the image is associated with the rare object responsive to the image being unrejected in a final detection stage of the first sequence, each detection stage configured under post-processing operation to propagate the image to a next detection stage, and record a rejection when the image is determined to be unassociated with the rare object, each detection stage capable of being configured under normal or postprocessing operation; and logic that determines a probability value that the image is associated with the rare object based at least in part on stage responses and stage decisions associated with the second sequence.