I first thought it was a camera but there was no point, After some searching I found that these things could communicate optical,, so that might be it, if so how do they work?
I'm pretty certain these ones use visible light, with ceiling-mounted LEDs. They flash at specific frequencies that are imperceptible to the human eye, but can be interpreted by the electronic shelf label. Here's a whitepaper explaining the concept.
Source: for 20 or so years, I worked in technology for some of Australia's largest retailers, and we tested a lot of these sorts of things in our labs. Very cool stuff.