Potential problems: trees, leaves, wind and sunshine (or clouds)
This bears a striking similarity to a traffic camera system produced by a local IT company I visited a decade or so ago at least. The aim was to let traffic cameras only record the passing cars, and stay shtum when there was no traffic. So a simple image differencing method was implemented, and if the sum of absolute differences between consecutive frames was sufficient, the system started sending a burst of frames, until the situation became static again.
Some traffic cameras, happily transmitted continuously from sunrise to sunset, especially on windy, sunny days, when the pattern of the shadow and light caused by the sun shining through the trees along the road caused loads of pixels to change, without any actual vehicle or person passing by the camera. Rushing clouds could trigger similar problems, as could snowflakes, hail, or rain. In the end they had to do far more advanced object recognition, in particular recognizing license plates to make the system robust (pedestrians and cyclists were not of interest in this system).
No doubt the boffins and DARPA will have thought of this