An apparatus comprises a projector for fixed attachment to a moveable part of an item of furniture, which changes its position and/or orientation relative to a surface in response to a movement of a subject occupying the item of furniture. The projector is arranged to project a pattern onto the surface. At least one camera is arranged to capture sequential images of the surface. A processing unit is arranged to: receive the images from the at least one camera； detect the projected pattern in the received images； detect a change to at least one feature of the projected pattern in the received images； determine whether the detected change satisfies at least one predefined criterion for a subject exit event； and output a signal based on the determining.
