Ultrasonic and PIR motion sensors will likely not work in your case, so
you'll probably have to go to some kind of computer vision scheme.
And probably will not be super easy to do.
I have read about people using simple CdS cells to detect motion [place
in a tube for directionality, and program A/D converter to sense "rapid"
changes], but these things are sensitive to all sorts of light-intensity
changes, ie "false positives". Welcome to the world of signal detection.