The Mystery of Cabin Island by Franklin W. Dixon

Frank was so close to the man that he recognized him readily. He knew now that Hanleigh was the man who had stolen their supplies and hidden them, evidently to get the boys to leave the island. He knew that Hanleigh was the man who had lost the mysterious Sparewell notebook. He wanted to know more. If he raised the alarm now, the man would simply refuse to talk.

Frank waited until the fellow had vanished among the trees. Then he turned and made his way toward the cabin by a shortcut. He wanted to reach the place first and warn Joe, so that they could better observe the man's actions without raising an immediate alarm.