That’s a difficult situation. Cats normally leave hedgehogs alone, so it’s unlikely to have been a cat. Some foxes do apparently become specialists at killing hedgehogs and if you have the misfortune to have one of those in the area or, if you have a badger in the area, it would probably be best to stop feeding the hedgehogs (if you are feeding them). Also is it possible that a dog could get into your garden?
Re. ultrasonic scarers. I don’t believe the manufacturers of those scarers are able to do tests with hedgehogs, so that there is no evidence that they do not also scare hedgehogs. I only have experience of a rat scarer in relation to hogs. I tried one once, years ago, just in the daytime and one night a hog turned up early before it was taken in and I saw it visibly flinch when it went past the repeller. So I would not use them with hogs around.