Well done getting help for the little one. I hope he is ok.
Most of the male hedgehogs here disappeared at the beginning of September with one remaining one disappearing 2 weeks later. They did the same last year and the first one returned at the beginning of March this year – about 2 months earlier than the females. It may not be quite as mild as Devon here, but probably not far off.
The females are slightly more complicated, as it is possible they are having late hoglets.
I know it is a worry, (some of the female visitors from here have also disappeared). There is no way of knowing why, but at this time of year, it may be a case of waiting until the Spring to see who turns up again.
Why they sometimes disappear earlier in the year is also a bit of a mystery. There could be many reasons other than that they have come to some sort of harm. For example, their route could have been blocked, someone else may be feeding them nearer, they may be having hoglets, etc. With wild animals it is not always possible to know. Even if they are in trouble, we can only help them if they come to us for help, like the little one there did. They give us joy, but also worry.