Thomas Orchard, 32, suffered a cardiac arrest in an Exeter police cell in 2012 and died a week later in hospital.
Sgt Jan Kingshott, 45, said Mr Orchard repeatedly threatened to "bite the faces off" custody staff.
Sgt Kingshott and detention officers Simon Tansley, 39, and Michael Marsden, 56, deny gross negligence manslaughter.
He told Bristol Crown Court: "It was unusual - a directed threat which makes you take notice. That's why it stuck in my mind."
Mr Orchard's mental health issues were not flagged up on the police computer nor was there a warning that he may be violent, the jury heard.
"I knew nothing about him - or what he was suffering from or his mental state or drugs," Mr Kingshott said.
Church caretaker Mr Orchard, who had schizophrenia, had stopped taking his medication when he was arrested for a public order offence and taken to the Heavitree Road police station.
He had a cloth emergency response belt (ERB) held around his face for five minutes to prevent him spitting and biting.
Sgt Kingshott told the jury he had seen the ERB used in this way on around 50 previous occasions and it was a method of restraint he had been trained in.
In initial interviews the custody sergeant told investigators he "had the impression that he [Mr Orchard] was actively resisting" but in subsequent questioning, after viewing CCTV pictures, he conceded "that perception is not correct".
The jury was told Devon and Cornwall Police has been investigated for corporate manslaughter and by the Health and Safety Executive following the death of Mr Orchard.
Those investigations are "substantially completed" with decisions to be taken in the near future over what will happen next.
The trial continues.
A police custody sergeant accused of the manslaughter of a detainee with mental health issues has told a court the man repeatedly threatened officers.