I used to work on automated surveillance. I can't wait until someone puts these services in the cloud to everyone will have access to the tools the feds are making. It actually doesn't make any sense that someone would be watching such a boring feed to catch the person in the act.

You need an intelligent DVR to find when the scene changes in a certain area, or an active system to detect if the sign has be taken. A pixel diff isn't enough considering wind, lighting changes, etc.