When I say that Nott Manor is one of the most haunted locations in Great Britain...I’m lying. It’s not one of, it is the most haunted.
The entire Nott family has a sick tradition of collecting dark artifacts and bringing them into the home. None of them collected at the magnitude that Coxius Nott did. With the encouragement of the Dark Lord himself, Coxius implemented Blood Wards into almost every room of the manor. He ensured that no Muggle or Muggle-Born would be able to step foot into the manor.
Locking the artifacts in what was once guest rooms, the dark magic literally poisoned not only the manor, but the land surrounding it. Theo is used to things moving on their own, being pricked or shocked or confunded in his own home.
By the time Theo leaves to Hogwarts at age eleven, he knows the concept of Blood Wards and his body is littered with small marks caused by dark magic. They will never go away, as magic like that never does leave.