This must work on the same principle as my garage light system which tells the wife to stop before going too close to wall but my system works with lights...
An alignment aid in a garage, like the safety system which stops a garage door from closing if something is in the way, might be based on a light
source and sensor in a fixed location: when something "breaks the beam" between them, that's the signal that the designated location has been reached.
The backup warning systems which have become common as new-car options (the $300 referred to in the Odyssey forum) are set of ultrasonic proximity sensors: they send out ultrasonic "pings" and listen for the echo, just like a crude sonar system. They can measure how far the object is from the sensor, and the alarm is just a comparison with a set distance. You could have a reading (in centimetres or inches or whatever) from these things if you wanted... they're not triggered by a very specific location.
The product described here looks like an ultrasonic proximity system.