Any sort of  surveillance  is essentially a monitoring of behavior. This could be done in an active sense such as when spying undetected, or in a passive sense when one is trying to guard an area in plain view. In a sense, surveillance equipment can be used both ways. They can be made obvious enough so that any unwanted behavior is deterred, or discreet enough so that people will act naturally. These systems are necessary in places where security is desired for one reason or another. A whole industry has sprung up regarding the manufacture and sale of equipment for this task.