Enid Blyton 
Secret Seven Adventure 
Secret Seven #2

Playing a game of American Indians in Little Thicket, beside a big mansion called Milton Manor — Colin climbs a tree hiding from the others, and whilst waiting to be found he sees a man on the wall of the mansion, and not long afterwards that very man is climbing up the tree Colin is hiding in. What on earth could be going on? The Seven soon learn of a stolen valuable pearl necklace. Could the man hiding in the tree have anything to do with this robbery? The Seven investigate, and discover unusual clues. A cap, funny marks in the earth near the wall. Will they find the thief following these odd clues?