Death caps are not in season right now in AU, correct?
If she dehydrated death cap mushrooms in error a while back and then she fished them out of her mushroom supply for the purposes of the beef wellington, her whole family would have long since been dead. In her ignorance, death caps would have contaminated everything a while ago.... the dehydrator, the knives, the cutting board, the masher, the counter top, other mushrooms, silverware lying adjacent.... she wouldn't have known to be mindful.
If she knowingly used death caps, she would have been mindful about where the toxins would land, and she wouldn't have allowed them to stray far, otherwise she'd be killing people at random.