"You have to be like Caesar's wife", said Today presenter John Humphrys this morning to Sir Hugh Orde, president of the Association of Chief Police Officers, the 'you' being the police in general (listen again here for another week). What is meant by 'like Caesar's wife' is 'above suspicion'.
The wife in question is Pompeia, Julius Caesar's second wife (his first wife Cornelia died in childbirth in 69 or 68 BC). The Roman writer Plutarch wrote in his Life of Caesar how Pompeia once hosted an all-women event, the Bona Dea festival, at her and Caesar's home. A man dressed as a woman surreptitiously gained entry. It is assumed that he was having an affair with Pompeia. He was caught and sent for trial for committing sacrilege by trespassing on a Vestal Virgin celebration, but was acquitted. Caesar did not come out and accuse his wife of adultery, but divorced her, nevertheless, on the grounds that his wife ought to be above suspicion.