Hmmm... I wonder what they're actually called - would help in the search.
You could get a security PIR sensor - the sort normally used to operate floodlights - set it to it's shortest duration (usually ~5 seconds) and connect it to a buzzer. Would also detect pedestrians.
You might need to blank the sides of the sensor so it only 'sees' a narrow sector to avoid false triggering though.
mab