An apparatus for detecting when a subject has exited an item of furniture is provided. The apparatus comprises a camera adapted to be arranged, when in use, below the underside of an item of furniture having an upper side for supporting a subject, the underside being opposite to the upper side, and a processing unit. The camera captures sequential images that do not include the upper side of the item of furniture, the images having a foreground corresponding to a region below the underside of the item of furniture and a background corresponding to a region adjacent the item. The processing unit receives images from the camera； detects, for each image, an edge corresponding to an edge of the item； detects the appearance of a feature contiguous with the edge； monitors how a detected feature changes over a time period； determines whether a change to the detected feature satisfies at least one predefined criterion for a subject exit event； and outputs a signal based on the determining.
