If the hedgehog almost curled up, it must have upsest it, Jens. So it might deter a hedgehog from visiting someone’s garden, if not away from a robo-mower. (if someone used a cat scarer) I personally wouldn’t want to use something which would upset the hedgehogs.
I can see what you are trying to do, to find something to keep hogs away from, for instance the robo-mowers to protect them. The problem for hedgehogs is that their natural defence is to roll up rather than to run away, so it seems unlikely that you would scare them away like that. It seems to me the only solution to the robo mower problem must be the construction of the bodywork of the actual mower. Maybe they could be built in such a way that they pushed any hedgehog out of the way without it being able to make any contact with the blades. There must be someone who could design such a thing.
