The Third Key (1956)

Someone has broken into the safe of a London company, and yet no force or explosive was used, and the keys do not appear to have been lost. A police superintendent is investigating, assisted by a new and very keen sergeant. The superintendent's wife is worried that he may be in personal danger, and the superintendent is not always mindful of the need to reassure his wife. Then a safe in another business is also robbed without the use of force, but the robber's getaway car runs down a bystander, who later dies. There seems to be nothing to link the crimes, but then the getaway car is found abandoned in a scrap yard. A discarded newspaper in the car may be a crucial lead in solving the crime, but there isn't much to go on.