Until not too long ago, a parent's US passport could be issued to cover
their young children traveling with them as well so the children would not
require or be issued their own separate passport.
Could it be that your mother was included on your grandmother's passport
and your grandmother used this passport for a trip only she was taking?
If so, the passport could be the reason your mother and well as your
grandmother might have been captured on some record during a journey
that only your grandmother was taking.
James Castellan, Rose Valley, PA USA firstname.lastname@example.org