Question

Does anything special happen when you find all Hidden Grottos?

Answer 1

The "Hidden Grotto Adept" Medal is given to players who have visited every Hidden Grotto.

Medal (Bulbapedia)

Answer 2

Nothing special really happens.

Source: Experience

Answer 3

I've done research on it and I didn't find anything on it so no, nothing special happens if you find them all. Sorry if you were hoping for something.
Below in the links are also listed the items and Pokemon you can find and where you can find them.
Hope I helped.

Source: Pokemon wiki, Bulbapedia, Serebii

Comments

I think you get a medal though ( ;