1. A method of reducing false positive security alerts, comprising: at a computing device having one or more processors, and memory storing one or more programs configured for execution by the one or more processors: computing a single depth map for a scene monitored by a video camera using a plurality of IR images captured by the video camera; using depth discontinuities in the single depth map to compute a depth edge map for the scene, wherein the depth discontinuities correspond to depth differences at adjacent points exceeding a predefined depth difference threshold and the depth edge map specifies lines of depth discontinuities in the depth man; identifying a first region within the scene that is enclosed by lines of depth discontinuities in the depth edge map; monitoring a video stream provided by the video camera to identify motion events, excluding the first region, and generating a motion alert when there is detected motion in the scene outside of the first region and the detected motion satisfies threshold criteria.