Often what may happen is that a clan will reappear in a province that is/was designated as Ronin. Usually it is confined to clans that still had an heir yet to be 'born' of great importance (check the new Imagawa daimyo, is it Tokugawa?)
You can expect roughly a 200% drop in loyalty when a daimyo dies, so it really does pay to have shinobi in every province; I used to not follow this last part, but now do because of situations similar to yours.