Hidden From View: Concealing and Revealing In Sandplay Therapy written by Debbi Mego, PHD, LCSW, CST-T is an excellent article in Journal of Sandplay Therapy, Volume 26 Number 2 2017.
In the article the "theme of what is hidden from view in the tray is explored as a transitional space between that which is completely buried or unconscious and that which is visible and conscious."
There is a discussion of "the symbolism of the veil and masked figures, which are inherently hidden." Through the exploration of the meaning and symbolism of this transitional space, sandplay therapists "can gain awareness of themes that are ready to come to the surface; this in turn helps them trace and guide their clients' progress in healing trauma".