Professor John McNeill has recommended changes to procedures and additional training at E Division of Police Scotland following the death last year.
The 37-year-old man found dead, who suffered from mental ill health and had a history of self-harming, was a patient at the Royal Edinburgh Hospital.
He was reported missing by the hospital last September 8 when he did not return from a weekend visit to his mother's home.
Police did not force entry to his Edinburgh home until six days later. It was clear the man had been dead for some time, though the date and time of death has not been determined. There were no suspicious circumstances.
The report by Professor McNeill, the Police Investigations & Review Commissioner, found it should have been apparent the man was a vulnerable person and the inquiry should have been graded as higher risk rather than low risk, as it was.
His report said police should have forced entry to the man's home on September 10 after his mother voiced "significant" concerns.
Mr McNeill said: "In this case there were repeated failures to update the Police National Computer with appropriate warning signals, which would have informed the missing person inquiry."
He also recommended that whenever people are reported missing from Royal Edinburgh Hospital police should make a physical visit to the hospital to ensure they obtain all the appropriate background information.
Chief Superintendent Mark Williams, local police commander for Edinburgh, said: "Since this incident was reported, the division has made several changes to protocols to ensure they align with national procedures."