Three Cheers Secret Seven by Enid Blyton
 
 Jack's sister, Susie, has a lovely new aeroplane, and the Secret Seven, and Susie go off to fly the new plane. Sadly it flies across the field, and over a wall into the grounds of Bartlett Lodge, which is a very big house but is closed up whilst the owners are abroad. So once the gardener is out of sight, Peter and Jack climb into the grounds of the large house and Peter climbs a tree to retrieve the plane which had landed on a high balcony. Just before he climbs down he sees a gas fire on inside one of the rooms, which he knows is strange as the house is empty. Feeling puzzled, Peter calls a meeting and their next mystery is about to begin!