Scout: The Haunted Castle by Piet Prins
Tom and Scout are on vacation with Tom's two best friends, Carl and Bert, and his sisters, Ina and Miriam. On their rambles through the woods, the group of young people are stopped one day by customs officials. They are accused of being in league with a gang of criminals who are smuggling goods across the nearby German border. Scout's reputation as a tracking dog has preceded him, and soon the young people are helping both the police and the customs officials. The mystery of the haunted castle, a coded message hidden under a tree, a gang of smugglers that can vanish at will, a burglary without clues — they all come together when Tom and Scout tumble over a four-hundred-year-old secret. Suddenly, however, Tom is no longer looking for adventure but fighting for his very life.