From the picture that you have included here, it is clear that you are asking about Chapter 3 of Call It Courage. In that chapter, the first clue that warns Mafatu that people have been on the island recently is found on p. 46. There, Mafatu has just found the altar where the cannibals have sacrificed human beings. We can tell that the cannibals have been there recently because the bones have not been there long. The book says that
Around the base of the idol he saw piles of bones, charred, but not old.
The next clue is closely related to this one and is found on p. 48. We are told that he could tell that the cannibals had been there recently because there had not yet been enough time for the wind to disturb the piles of ashes left behind by their fires. As the book says
It was evident that the savages had been here recently, for the piles of ashes rested undisturbed by wind and storm.
Thus, both of the clues that tell Mafatu that people have been on the island recently are related to the human sacrifices carried out by those people.