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 that determines whether an image is associated with a rare object, each detection stage under normal operation rejects the image when the image is determined to be unassociated with the rare object and propagates the image to a next detection stage when the image is unrejected; a second sequence of two or more detection stages that determines 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 under post-processing operation propagates the image to a next detection stage and records a rejection when the image is determined to be unassociated with the rare object, wherein each detection stage is configurable under normal or post-processing 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.