Metila Norwood worked for the British intelligence Service. She was also a KGB mole recruited in 1935. She penetrated the British secret Service establishment and was responsible for handing secrets to the KGB for many years.
Much of the information passed to the Soviets was very valuable nuclear technology secrets that she got from her boss's safe. She would photograph them and then discretely return the original files back into the safe.
Read her story and how she was discovered in the Guardian newspaper link below: