Sorry not on Facebook, so can’t see it. But it does sound like what I was saying. It looks a bit as if they are scent marking, but apparently they don’t have scent glands there, so possibly that isn’t the real reason. Maybe they’re just scratching/rubbing their chins/necks/underneath.
There is a lot about hogs that we still don’t know for sure, but you will find that sometimes theories are stated as facts, which isn’t ideal. There can also be some confusion between the behaviour of our wild European hedgehogs and the African Pygmy hedgehogs which some people keep as pets. i.e. some sites are talking about those, rather than the wild Euoropean hedgehogs.
Not sure about the stretching out to cool down thing. If they (wild European hedgehogs) were behaving like that during the day, it would suggest something was wrong with them. They could be unwell.