Xanthes decides to go carefully investigate the place where the prisoners are held then (the dungeons, presumably). How do they get their food, who brings it, and when? Is there any place in this chain from kitchens to mouths that she could easily and surreptitiously intercept the food meant for the prisoners? She figures it's more likely the guards serve the prisoners than servants do. But surely servants have to deliver that food to the guards? She will find out.