This is sort of along to the same theory...
Would it be possible that Snape's task is to go and impersonate a Dementor, using Polyjuice potion?(if they have hair, lol) If so, I think that would make sense. There are tons of them so it's easy to fit in, and would be hard to test each and every one of them for polyjuice. 
Just a thought...
~Echo

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