1. A method of video surveillance, comprising: receiving, by a computer system, a video comprising video images from a video sensor, the computer system performing the steps of: analyzing the video to detect stationary objects in the video using background subtraction of a background model; analyzing the video to detect people in the video by using frame differencing to detect foreground objects; upon detecting a first stationary object in the video, defining a zone as a portion of the video around the first stationary object, defining the zone being responsive to a location of the first stationary object, the zone being defined to be larger than an outer boundary of the first stationary object and smaller than a field of view of the video, a size of the zone allowing detection of multiple non-overlapping objects; tracking a duration that the first stationary object remains stationary; determining an occurrence of an external event; issuing an alert in response to determining that the duration of time the first stationary object has remained stationary exceeds a threshold while no person of interest detected in the video has been inside the zone and the occurrence of the external event; and determining a crowd density in the video, wherein the size of the zone is determined dynamically and is responsive to the determined crowd density.