Well you know my contests, I never give you all the info, I like to see how crazy and outrageous people get with that noodle between their ear bones.
Its actually much simpler than a sobel filter, it was actually designed for motion detection.
I did a time lapse historical average, with full storage of the last 100 frames, then I XOR'd it with the current image. Areas that didnt change would have zero values, areas that did change would have non-zero values.
Here is another one -
