Officers found the a 52-year-old man unresponsive in a cell at Croydon custody centre.
West Yorkshire Police refers crash where two teenagers were injured to watchdog.
The officer is due in court after a man was allegedly kicked unconscious during an arrest in Leeds.
The Independent Office for Police Conduct is investigating four Hampshire constables.
Ricky Swindells, 28, was "in crisis on the floor" when he was restraining by GMP officers.
The police watchdog says a police officer is being investigated for potential gross misconduct.
Youth panel members share ideas with chief constable on improving custody system for young people
The probe will cover the circumstances leading up to the death of a 16-year-old boy in Pontefract.
A police watchdog says the man in his 40s was arrested in Blackpool town centre on Saturday evening.
