Motion Tracking Using Arduino?
You need the person to emit a signal and the robot to receive it, sounds simple, but it isn't.  What you are trying to do is position fix the transmitter, in the same way as an ATC locates a plane.  

The robot will need at least two directional receivers positioned a certain distance apart (depends of frequency and accuracy) to allow the robot to triangulate the transmitter.  The more receivers the more complex the triangulation.  The frequency of the signal you transmit will affect the distance that the robot can be from the person, research Frequency spectrum and transmission distances.  The longer the distance the signal can travel the stronger the signal will be at close range.  You also have to counter reflections and side lobes which will give you inaccurate readings on the transmitter direction.
Once you have dealt with all of that all you need to do is scan for the strongest signal on your frequency, triangulate it and then head towards it.
Whatever sensor you use the theory is the pretty much the same.

Could you tag the person with a GPS receiver and WIFI/BT and get it to transmit its location.  The robot could then consult its GPS and calculate the delta and move accordingly.