Or if they're brought back by a necromancer. [Which is a disturbing possibility, but one Sabriel regards as unlikely. These ghosts aren't attacking anyone, or acting in a way that suggests they're all under the control of a single will. But no pages of the Book of the Dead surface in her memory to provide further information, so Sabriel presses on.]

But in this case, that's unlikely. A necromancer would try to use them in some way, not just leave them to their own devices.