I hadn't checked the rat box for a couple of days. Overnight a hard frost and a light cover of snow left a very different-looking site. The box was empty, but there were no footprints in the snow at all, so the poison must have been taken before the snow fell. However, how did they know not to return last night?
I added some more poison and walked around the site. There were no vehicle tracks in the car parks and no footprints to be seen. I quite like the effect of the snow and the frost might curtail the weeds for a while.