1. A method for detecting an object left-behind in a scene, comprising the steps of: updating a set of background models using a sequence of images acquired of a scene by a camera, each background model updated at a different temporal scales ranging from short term to long term; determining a foreground mask from each background model after the updating for a particular image of the sequence; updating a motion image from the set of foreground masks, in which each pixel in the motion image has an associated evidence value; and comparing the evidence values with a evidence threshold to detect and signal an object left behind in the scene.