I would keep a eye on the amount of poison that is down..belive this or not i trapped around 36 mice in my mother's house several year's ago..i got sick of catching them in the end so poison was used..the poison just kept going..in the end i used 6 bag's of poison before it stopped beeing taken and the problem was solved..
Plus with rat's and the older type poison im sure they have to eat it over a period of time for it to be effective and i would not rule out that they maybe more than one..