Thanks for your answer! Yeah, it seems to be a speck of dust. It's exactly where you mention, between the layer and the screen, but it has to be firmly rooted because I tap the screen and the speck remains in the same spot. Thus, it seems that the speck cannot be removed by the user..that's a shame
Given that it does not affect the overall experience of reading (even if it distracts me from time to time), I will have to learn how to live with it, unless the issue gets worse (say, a boom of dust specks around the screen!), forcing me to look for Sony's support here (a scenario I do not want to live!). Thanks again!