If you are talking about an actual hidden test case that isn't doing enough tests, then you will have to specify which programming language you are talking about.

The Sample Test Case is one you are supposed to write yourself. And it is not like this kata is all that hard, so it feels like overkill to add all those.

The basic idea I had when I made it this kata, was to have users learn that they can't trust the Sample Test Case to be complete and they should expand it themselves if they think they need it.

If you really want to test your solution, you would submit it anyway, which has all the hidden test cases.