A police car with siren sounding pulls up in front of Rose's house and a policeman calls at her door to inform her that her juvenile son has run away from the Work Farm and asks to search the house to see if he is hiding there. The police only use their car sirens for emergency situations and when in pursuit of other vehicles, etc. - not when making routine house calls. And even if her son were hiding at the house (which, in terms of the movie's plot, he wasn't), the blaring siren would have alerted him that he was being sought by the police, causing him to flee into the backyard to make his escape before the policeman entered the house.