Loiter alarm

My back door opens on to a busy alleyway. Having alarms every time someone passes by is not useful. However I would like it to notify me if there is continuous motion in the security zone that I set just outside my door if somebody is there for more than a number of seconds or minutes. This is to address people trying to force the door or sitting on my doorstep and or urinating on my door, which is probably my most annoying situation