I. Introduction
Consumer surveillance cameras are cheap and ubiquitous. The advent of smart consumer cameras with higher processing capabilities has now made it possible to design video surveillance systems which can contribute to the safety of people in the home and in public places such as shopping malls, airports, railways stations and etc. Terrorist attacks have become a critical threat of public safety; especially, explosive attacks with unattended packages are repeatedly concentrated on such public places. A key function in such a surveillance system is the understanding of human behavior in relation with objects left unattended in public places.