There seem to be three terms used by experts in the field However, the other two seem to be more commonly used in that context. Except non is not an english word, it is a prefix of latin origin Which is why american style manuals will always ask you to merge it with the subsequent word, without a hyphen In expressions in which figures and abbreviations (or symbols. Leaving non stranded doesn't work either as it is a bound morpheme, a prefix not a word (in english)
I'd use the two hyphens. When i'm pointing to something that is outside a predefined scope.
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