Cameras mounted outside sway about in the wind so trying to detect when something in the camera's field of view has moved and it isn't too big/small to be what you are looking for, or noise, or the camera moving is quite a problem and one which is related to work I do with ANPR cameras, I solved the problem of how you do resolve those problems and my company took out a patent on the algorithm.So I could tell you how to do it but you would have to pay me or my company, preferably me a lot of money.
The application was to detect moving vehicles when most of the camera was in sleep mode, so it just used an igloo FPGA and an image sensor, It could distinguish between crap blowing across the road, people walking over the road, the camera wobbling around on the pole and cars driving down the road.