There is provided an apparatus for detecting when a subject has exited an item of furniture which has an upper side for supporting a subject and an underside opposite to the upper side, and comprises an overhanging component that extends from the underside toward a surface on which the item of furniture is resting. The apparatus comprises a light source to illuminate an edge-feature associated with a distal edge of the overhanging component, to create a shadow of the edge-feature on a region of the surface. The apparatus further comprises a camera arranged to capture sequential images of the region of the surface and a processing unit. The processing unit is arranged to receive the images detect an edge of the shadow in each of the received images, detect, in the received images, the appearance of a feature contiguous with the shadow edge monitor how a detected feature changes over a time period determine whether a change to the detected feature satisfies at least one predefined criterion for a subject exit event and output a signal based on the determining.
