There is relevant data, but I don't know where it is, sorry.
I am under the impression that this sort of under-the-radar hiring occurs enough to be a significant cause of institutional racism in hiring. (If your existing employees are disproportionately white, and their friends and acquaintances are disproportionately white, then new employees who fill a position before it is publicly advertised will be disproportionately white; and this is without anyone being overtly or consciously racist, and without non-whites being discouraged in any way from applying.)
I am under the impression that this sort of under-the-radar hiring occurs enough to be a significant cause of institutional racism in hiring. (If your existing employees are disproportionately white, and their friends and acquaintances are disproportionately white, then new employees who fill a position before it is publicly advertised will be disproportionately white; and this is without anyone being overtly or consciously racist, and without non-whites being discouraged in any way from applying.)