1. A method for detecting one or more events, comprising: generating a motion graph from a video sequence captured at a point-of-sale (POS) system, the motion graph representing human motion in the video sequence; detecting two consecutive scan events at the POS system by identifying a first reference motion pattern in the motion graph; identifying a segment of the video sequence between the two consecutive scan events; and upon identifying the video sequence segment, analyzing a portion of the motion graph corresponding to the video sequence segment to detect at least one of a pick-up event and a drop event by identifying a second reference motion pattern in the motion graph.