Famous Five Five Go To Smugglers Top Enid Blyton # 4

‘Julian gave an exclamation. Someone was signaling from the tower!’                                       Visiting the old house at Smuggler’s Top is thrilling – there are hidden passages and underground tunnels to explore.
Then not one, but two, people disappear from the house and the children notice a mysterious signal out to sea.
What secret is Smuggler’s Top about to reveal?