A processing unit with at least one image that relates to a scene external to a vehicle, processes the image to extract low level features. A low level feature is defined such that an object in the scene cannot be characterized on the basis of at least one low level feature in a single image of the at least one scene. The processing unit determines if at least a part of an object is present in a part of the scene, the determination comprises an analysis of the low level features. An output unit outputs information that there is something present in the scene.