Pokémon: Sun & Moon

Sun seeks riches, while Moon desires knowledge, but both of their goals are impeded by the interference of Team Skull.
As Moon travels to deliver a rare Pokémon to Professor Kukui in Alola, she meets Sun, who is entrusted with a one-of-a-kind Pokédex. Sun offers to accompany Moon on her journey, but they are soon ambushed by Team Skull and must face a perplexing attack from an unknown Pokémon.
What clue does this enigmatic creature leave behind to indicate its whereabouts?