Three cheers for the seven - The seven 8

The Secret Seven are getting ready for another meeting, but there's no case to solve. Since Janet, Jack's annoying sister, received a spectacular plane that everyone wants to see fly, they decide to invite her for a picnic and to play.

Accidentally, the plane goes over a high wall and lands on a balcony at Bartlett Lodge. Jack and Peter decide to retrieve it and, through a crack, Peter sees the light of a gas heater.

If Jack's parents had said the house was uninhabited because the owners were away, who could be inside?

It's another case that only The Secret Seven can solve!