Abstract: An abnormal-occurrence-detection-system comprising an abnormal-occurrence-detector inspecting a plurality of inspected-data-instances, each data-instance including values associated with at least one physical-attribute, the values defining the location of each data-instance in an attribute-space, the abnormal-occurrence-detector detecting when at least one data-instance corresponds to an abnormal-occurrence according to one of the following: when a density-point associated with one of the inspected-data-instances is not associated with at least one hilltop-point, and when the distance in the attribute-space, between a selected one of the inspected-data-instances associated with a first respective unique-identifier in a sorted list of unique-identifiers and a respective Kth adjacent inspected-data-instance associated with a second respective unique-identifier in the sorted list of unique-identifiers, exceeds a distance-threshold-for-Kth-adjacency, the sorted list of unique-identifiers defining a sorted sequenc