They’re trying to detect anomalies:

If you have ever wondered how security guards can possibly keep an unfailingly vigilant watch on every single one of dozens of television monitors, each depicting a different scene, the answer seems to be (as you suspected): they can’t.

Instead, they can now rely on computers to constantly analyze the patterns, sizes, speeds, angles and motion picked up by the camera and determine — based on how they have been programmed — whether this constitutes a possible threat. In which case, the computer alerts the security guard whose own eyes may have been momentarily diverted. Or shut.

An alarm can be raised, for instance, if the computer discerns a vehicle that has been standing still for too long (say, a van in the drop-off lane of an airport terminal) or a person who is loitering while everyone else is in motion. By the same token, it will spot the individual who is moving rapidly while everyone else is shuffling along. It can spot a package that has been left behind and identify which figure in the crowd abandoned it. Or pinpoint the individual who is moving the wrong way down a one-way corridor.

Because one person’s “abnormal situation” is another person’s “hot dog vendor attracting a small crowd,” the computers can be programmed to discern between times of the day and days of the week.