1. A portal security system, comprising: a sensor capturing plural 2D images of a field of view about a portal; a 3D imaging system generating a target volume from the plural 2D images of the field of view about the portal, the portal defining a moving volume through which a person passes from an entrance to an exit; and a processor dynamically varying a position of the target volume within the field of view as the moving volume moves through the field of view, the processor varying the position to correspond to the moving volume according to a position of the portal, the processor detecting and tracking people candidates moving through the target volume to detect a portal access event.