I'm trying to help out a friend with something. Is there any expression for when something has been done, but nobody knows whom by? In Dutch there is an expression which translates into "the gnomes must have done it".
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There are very similar phrases in English, mostly related to various fairy tales borrowed from around the world. For instance, "The elves must have done it" is a commonly-used reference to "The Elves and the Shoemaker", which I believe is Dutch in origin and very similar to your quoted phrase. References to "your fairy godmother", "fairies", and gift-givers like Santa and the Easter Bunny also abound in English culture, even far from their traditional holidays. "Christmas in July" is a common term for an unexpected abundance, whether through generosity or providence, and it doesn't have to be in July.
On the other hand, when something bad has happened inexplicably, "gremlins" are a common scapegoat, especially in the context of complex systems such as aviation and computing.