1. A method comprising: at a computing system having one or more processors and memory: obtaining a plurality of video frames, the plurality of video frames corresponding to a scene and a motion event candidate; identifying one or more visual characteristics of the scene; obtaining one or more background factors for the scene; utilizing the obtained background factors to identify one or more motion entities; for each identified motion entity: classifying the motion entity by performing object recognition on the motion entity; and obtaining one or more representative motion vectors based on a motion track of the motion entity; and assigning a motion event category of a plurality of motion event categories to the motion event candidate based on the identified one or more visual characteristics, the obtained background factors, the classified motion entities, and the obtained representative motion vectors; wherein the motion event category assigned to the motion event candidate is selected from a group consisting of: one or more known event types; one or more unknown event types; and a non-event type.