Activity is an awfully vague term. I know of one company that's hired professional actors to act out different scenarios, in the hopes that they can distill th

Activity is an awfully vague term. I know of one company that's hired professional actors to act out different scenarios, in the hopes that they can distill the essence of suspicious activity, and thereby automatically detect it. I'm not optimistic about their chances. However, it's relatively straightforward to detect people tailgating at security entrances (using one passcard to let two people in), or walking the wrong way into the exit gate at an airport. In both these situations, though, the environment is controlled, and the difference between proper and improper behavior is well-defined. Neither of these things are true on, say, a street corner. Human interaction and body language is complicated. I expect we'll have a good way of automatically detecting behavior sometime around when we have general artificial intelligences, and not before.