After over forty years of being closely involved in the observation, capture and shooting of all kinds of feathered, furry and other creatures, I have come to the conclusion that the population of particular species follows a series of peaks and troughs, not necessarily regular. I agree that this year, and last year for that matter, I have seen fewer hedgehogs around, or indeed evidence of their activity. However, in my part of the world at least, there are far more rabbits than last year, more deer and certainly more badgers. (Very few grey squirrels spotted so far this year.....not a bad thing!).
Four or five years ago, I saw little or no evidence of rabbits locally, and certainly, the falconer I sometimes accompany reported few rabbit kills.
I think many things like this in nature are cyclical and unless the decline is sustained over four or five years, then it means little other than mother nature naturally controlling numbers.
I suspect that hedgehogs will make a renewed appearance in the next year or two.