PaulS:
They use ultrasonic sensors which are better for short-distance detection and more directional than radio.
But can't discriminate between following you or some pretty chick that walks by. Well, I'd follow her, too.
If you had looked at the project you would have seen that the person being followed had a radio-triggered ultrasonic sender and the luggage had two ultrasonic receivers and a radio sender. The luggage radio commands the transponder on the human to send out a ping. The two receivers time the transit of the ping. This gives distance to the transponder and relative direction, enough to maintain a constant follow distance.