Creature of Cassidy's Creek

There's a weird noise in Gran's cellar, so Sebastian bolts the door. In the night he hears another noise, and finds the door ajar. next morning, he notices a half-eaten apple, a split vase of flowers, and a curtain hanging off its rail. Later, the bolt on the celler door is locked again, but Gran explains that she closed the door. Sebastian climbs a tree and sees movement in the front window of the house. then he discovers a possum in the cellar. the mystery is solved, but Sebastian realises that while he was up in the tree the possum had been locked in the cellar - so what could the movement in the window have been? (The answer is revealed in the pictures on pp.19 and 32 - There is a second possum in the house!)