Have you looked at some of the Passive Infrared detectors? Here are some available from Newark.
My neighbor has one mounted on his house and it turns on his lights every time I walk by 20 meters away.
If you want ultrasonic distance measurement, Maxbotix has units that measure up to 10 meters.
There are also some low cost video camcorders that have built-in motion detection, but it is not clear what would be useful for you.