1. A method of detecting background changes in a visual scene, the method comprising the steps of: capturing images of the scene into which objects enter and from which objects leave; generating a first probabilistic measure of the scene indicative of a relatively static background of the scene; generating a second probabilistic measure of the scene indicative of a relatively dynamic transient objects entering and leaving the scene; and detecting whether the second probabilistic measure deviates from the first probabilistic measure, indicating that an object entering the scene has not left the scene.