The Secret Warning by Franklin W. Dixon

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    Frank related their theory that the pearls might have been the object of the burglary attempts and told how they had found the cane to be empty.
    â€œDon’t worry. I haven’t carried any pearls in the cane since I retired from the Service,” Captain Early replied. “You fellows deserve credit for a smart guess, though.”
    The captain explained that the pearls which he had collected had been made into a necklace for his late wife, and now were owned by one of her relatives.
    Frank, somewhat letdown, observed, “Even so, the burglar must have thought the cane still held a fortune in pearls, just as we did.”
    â€œHmm. I suppose that’s possible,” Captain Early agreed, “if he’d heard about me from some acquaintance in the Navy.”
    â€œOne thing has us stymied,” Frank went on. “How did he know the cane was still at our house? If he trailed you there Monday night and saw you leave the next day, I should think he would’ve been fooled by seeing you carry Dad’s walking stick.”
    â€œHold on! Maybe he was!” Captain Early said excitedly. “You remember my telling you about that motorist who picked me up?”
    â€œYes.”
    â€œThe fellow seemed interested in my cane—even asked to take a look at it after I got into his car. It was then I first noticed I’d taken the wrong one, and I mentioned the mix-up.”
    â€œWow! That could mean he drained the gas tank Monday night!” Frank exclaimed. “He may have counted on picking you up when you ran out of gas, swiping your cane, and pushing you out of the car!”
    Joe, who was listening in on the conversation, broke in, “So he knew where to look for the cane—at our house.”
    â€œWell, boys, your theory seems to explain all the angles of the case,” Captain Early said. “At any rate, the burglar hasn’t come back. And I hope he doesn’t.”
    Frank ended the call after getting a description of the motorist and his car.
    The rain ceased and the skies began to clear soon after the boys emerged from the radio shack. Roland Perry met them out on deck.
    â€œLooks as though we’re in luck,” he remarked. “Captain says the hurricane’s moving out to sea again.”
    â€œHmm! That air sure smells good,” said Chet, who was rapidly regaining his usual healthy appetite. “Think I’ll go see if the cook has any leftovers.”
    â€œWatch it. He may put you to work washing dishes,” Joe joked.
    â€œWho cares? It’ll be worth it!” Chet said breezily, and trotted off toward the galley.
    Stars were now twinkling brightly and the cove lay silvered with moonlight. Voices carried across the water from the Salvor anchored nearby.
    Perry eyed the other boat suspiciously. “I’d sure give a lot to know what those bilge rats are after.”
    The Hardys, recalling Bock’s threat to their father, expressed the same interest. Frank then told the diver about the cave on the island. “It looks as if somebody lived in it.”
    â€œAnd it may be the answer to a ghost mystery,” Joe stated. “You want to have a look at it?”
    Perry, intrigued, quickly agreed to accompany the Hardys ashore. The Sleuth was lowered over the side and a few spurts of her motor brought them quickly to the beach.
    When they reached the cave, Frank led the way inside. He shone his flashlight beam over the campsite traces on the floor, then upward to the tally marks scratched on the wall.
    â€œPoor guy. Must have had a pretty rugged diet,” said Perry, toeing the scattered bird bones. “I’d say he was probably a shipwrecked sailor or a stranded fisherman.”
    â€œIn that case, why live in a cave when there’s a perfectly good lighthouse handy?” Joe countered.
    â€œHmm, you have a point there.” The diver rubbed his jaw thoughtfully. “What’s this ghost mystery