Whenever you take 256 steps in the game, there is a 5% chance that
each of the Hidden Grottos will be regenerated. When it's regenerated,
there's a 40% chance of an item, a 40% chance of a hidden item and a
20% chance of a Pokémon. The chance that it regenerates can be
increased using a Pass Power.
Actually if you're really lucky, you might get pachurisu every time you walk 256 steps