1. A video monitoring system comprising: a camera unit comprising a camera arranged to monitor a scene, a motion sensor configured to detect motion in a plurality of predetermined scene regions, and a video encoder comprising a first data input configured to receive a video stream with a plurality of image frames from the camera, a second data input configured to receive sensor data from the motion sensor, and a processing circuit configured to generate an overlay representing the sensor data and add the overlay to the image frames in the video stream to generate an encoded video stream which represents the scene in two different ways in every image frame, wherein the overlay has a plurality of overlay regions each representing a corresponding scene region, wherein the overlay includes a graphical element in those overlay regions which represent scene regions in which the sensor data indicates that motion was detected by the motion sensor, wherein the camera is configured to output the encoded video stream generated by the processing circuit; and a video management system comprising a third data input configured to receive the encoded video stream from the camera unit, and a video analytics circuit configured to analyze the encoded video stream and to generate an event when an object is detected in a scene region and a graphical element is detected at the same time in an overlay region representing that scene region.