Police were only alerted that a mental health patient was missing at least five hours after she was last seen by staff, an inquest has heard.

Shirley Cassman, 68, was found in a river off Brunden Lane in Sudbury on December 13, 2012 - 48 hours after she left the Southgate ward of the Wedgewood Unit in Bury St Edmunds.

Ms Cassman, who had a history of anxiety and depression, had been admitted to the unit - which is run by Norfolk and Suffolk NHS Foundation Trust - on December 4 as a voluntary patient following a downturn in her mental state.

Yesterday, a full inquest into her death heard she left the unit some time between 9am and 10am on December 11 to go to the shops - which she had permission to do - but it was unclear among staff whether she had gone to the shops at West Suffolk Hospital, which is on the same site, or those in the town.

It was reported to mental health nurse Martin Kemp, who was in charge that day, at 11am that Ms Cassman, from Melford Road, Sudbury, had failed to return, but it was not until after 3pm that the police were contacted to help with the search.

Mike Seaman, acute services manager at Norfolk and Suffolk NHS Foundation Trust, told the inquest an internal investigation was held following her death and a number of changes had been made as a direct result of her case.

Now, there would be a very quick search of the ward, and hotspots such as the restaurant, then the perimeters of the site on the way back and the police would then be contacted.

“The policy back then we would identify was not good enough,” said Mr Seaman, who is a registered mental health nurse.

Coroner Yvonne Blake made a narrative conclusion, saying she did not think there was any evidence that Ms Cassman intended to take her own life.

After the inquest, which was held in Bury St Edmunds, Ms Cassman’s son, Jason Watt, from Wiltshire, said: “The tragedy of what happened to her was a catalyst for change. That was essential and necessary within that trust and within that unit and it comes as a great comfort to us that change happened quickly at least.”

Ms Cassman’s family had also raised communication with relatives as an issue. Mr Seaman said better practices had been identified, adding one of the changes introduced were ‘carers clinics’ which give carers the chance to meet with senior ward staff on a Tuesday and Thursday evening.

The inquest heard of the efforts, including from a volunteer sniffer dog team, to locate Ms Cassman, who was eventually found by Police Community Support Officer Mick Baxter, of the Sudbury and Great Cornard Safer Neighbourhood Team.

The inquest heard the cause of Ms Cassman’s death was vaso-vagal inhibition - or ‘dry drowning’ - with immersion and also disorientation due to Metazapine and Zopiclone intoxication noted as factors.

Independent pathologist Dr Ian Calder said there was evidence there had been the sudden inhalation of a small amount of cold water, which he referred to as dry drowning.

Ms Cassman, who had previously self-harmed and had had suicidal thoughts, had worked with children in care homes before she retired.