Hedgehogs can travel quite a long way – the males 1-2 miles in a night, so if someone is marking them, they may not be in the immediate neighbourhood. It would seem strange that 9 different hedgehogs would have patches on them otherwise. It is not normal for hogs to have white patches on them naturally (although they do sometimes have 2 or 3 individual pale spines), but it seems to be becoming more common for people to mark them, so that would be my guess.
You are lucky to have lots of neighbours who are happy to help the hedgehogs. Sounds an ideal place for them with all those gardens to visit.