From a 2014 article on Business Insider http://www.businessinsider.com/how-retailers-track-shoppers-in-heat-maps-2014-1

A company called Prism Skylabs (ominous!) (and their website doesn’t load now…) uses cameras (and computer vision?) to track where people walk, linger, and interact in a store. The pitch is that the info can be used to optimize product placement – if something isn’t interesting, move it to a back corner. If something is interesting, make it more prominent. Etc.