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, who suffered from mental ill health and had a history of self-harming, was a patient at the Royal Edinburgh Hospital (REH) and was reported missing by the hospital on Sunday September 8 last year 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, on September 14, after his mother had raised "significant concerns".
It was clear that 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 Prof McNeill, the Police Investigations & Review Commissioner (PIRC), found that it should have been apparent that 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 that Police Scotland should have forced entry to the man's home on September 10 after his mother voiced major concerns.
Prof 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.
"This case highlights the importance of accurately recording all available information on missing persons and making that information available to operational officers.
"I hope that the recommendations I have made as a result of this tragic case will contribute to improvements in the police response and management of all vulnerable missing person enquiries."
Prof McNeill also highlighted "a lack of clear ownership and accountability" in respect of the missing person inquiry, with numerous police supervisors endorsing what their colleagues had determined without undertaking a critical examination of all the available information and evidence.
The investigation also found that officers had followed the Missing Person Protocol between the REH and the former Lothian and Borders police force joint protocol ('E' Division protocol), rather than the Police Scotland missing, wanted & found persons, abscondees and escapees standard operating procedures (SOP).
As a result, officers did not obtain all available and relevant information following the initial missing person report and subsequently did not undertake thorough enquiries as required by the Police Scotland SOP.
They did not search the man's mother's address and did not ensure that medical staff searched the appropriate areas of the REH.
Prof McNeill's recommendations include that Police Scotland should consider providing additional training to appropriate supervisory ranks in E Division in the management and conduct of missing person investigations.
He called for the protocol followed in this case to be brought into line with the national missing person procedures.
He also recommended that whenever people are reported missing from REH police should make a physical visit to the hospital to ensure they obtain all the appropriate background information.
Assistant Chief Constable Wayne Mawson, the lead officer for missing person policy for Police Scotland, said: "We are closely examining the findings of the commissioner and thank him for his report.
"Since the launch of Police Scotland in April 2013 we have worked to ensure that missing persons investigations are conducted professionally and consistently across the country.
"We have introduced into every division a missing person co-ordinator. We have also created a national oversight group led by a Detective Superintendent.
"This group scrutinises ongoing high risk cases to ensure every opportunity is taken to trace those reported missing to us. Local police commanders have oversight of investigations in their divisions and these are monitored daily.
"We recognise that from the moment someone is reported missing to us, our response has to be the best to maximise the chances of tracing that person.
"The police in Scotland dealt with more than 32,000 missing person investigations last year.
"Each of those is unique in terms of its complexity and circumstances, and those reported missing to us are often vulnerable.
"We recognise that in this case the standards normally expected of the police have not been met.
"We will look carefully at the findings presented here and continue to modernise our approach to missing persons investigations to ensure we continue to keep people safe."
Chief Superintendent Mark Williams, local police commander for Edinburgh, said: "I acknowledge the findings of the commissioner and the recommendations he has made.
"Since this incident was reported, the division has made several changes to protocols to ensure they align with national procedures."