The police theory of the murder (and I was prepared to accept it) was that the assassin had been crouching in hiding behind or beside the cab--or even within the dark interior. He had leaped in and attacked the woman at the moment that the taxi-man had started his engine; if already inside, the deed had proven even easier. Then, during some block in the traffic, he had slipped out unseen, leaving the body of the victim to be discovered when the cab pulled up at the hotel.