If we examine the Mona Lisa face, zone by zone, the reason for its mysteriousness becomes clear: there are different emotions expressed in different facial zones.
Her mouth, as everyone has noticed, has a slight smile.
Her eyes are a little sad.
Her forehead is blank and unexpressive.
From an analysis of the Mona Lisa from the perspective of the social psychology of facial expressions (see Malcolm Gladwell's profile of Ekman and the field he originated, the study of micro expressions).
I wonder if Dan Brown can turn this into a page-turner.