A conventional explanation for self-charging is that temperature variations,
say from day to night in the room where the capacitor is stored, cause tiny
expansions and contractions of the capacitor materials. The friction of the
plates and insulators moving against each other generates charge.
I've tried rubbing a plastic rod between two sheets of aluminium, insulated
and also just held in my hand, and in both cases the rod got charged and
picked up pieces of paper.
Bruce.