The Hypothesis explain phase [1][2] does this!

  fails_on_empty_third_arg(
      a = "",  # or any other generated value
      b = "",  # or any other generated value
      c = "",  
      d = "",  # or any other generated value
  )

[1] https://hypothesis.readthedocs.io/en/latest/reference/api.ht...

[2] https://github.com/HypothesisWorks/hypothesis/pull/3555

The special value doesn't stand out, though. All three examples I gave were what I thought skimming his comment before my brain caught up to his caveat about an empty third argument. The empty string looked like it was by far the most harmless part... Whereas if they are all empty strings, then by definition the empty string stands out as the most suspicious possible part.