so the robot/bag knows to stop before running down the user
Oh Richard you have spoilt the surprise. 
Yes the sun will be an issue with IR detectors. You could try modulated IR. The best solution could be a GPS receiver on your belt transmitting your position to the bag robot. Then the bag robot has a GPS so it knows where it is and then it can plot a course to you.
P.S. In the UK the term Bag Lady is used to describe a female tramp or bum. So the term Bag Robot might have different meanings in some parts of the world.