Detectives reviewing her missing person file have all but ruled out Ms Barclay dying of a medical event, committing suicide or changing her identity.
That left one likely option for her disappearance: homicide.
''But without a body and without knowing what has happened to her, you can't say 100% whether it is a murder,'' Detective Senior Sergeant Malcolm Inglis said.
Ms Barclay was last seen at her Wakari home on September 17, 2002.
However, a date-stamped envelope shown to the Otago Daily Times by detectives investigating ''Operation Tui'' seems to confirm the then 29-year-old was alive nine days later.
That message, to her fiancee Bill Brown, indicated the relationship was at an end: ''Here [is] the ring back who wants to Marry a mental Bitch''.
Five months after the last confirmed sighting of Ms Barclay, she was reported missing by Mr Brown - on February 28, 2003.
Several years later, Mr Brown - a British passport holder - took their youngest two children, now aged 12 and 13, to live in England.
He had since made contact with detectives investigating the case, wanting ''to know what we are doing in relation to the inquiry'', Det Snr Sgt Inglis said.
''He has two children in the United Kingdom that are her children as well ... they have no idea what has happened to their mother ... they would have no idea who their mother was.''
Last week, Mr Brown, approached for comment by the ODT, responded that a media statement would be a ''fair response''.
But he has sent nothing, and has not responded to messages.
As part of their inquiries, police have already searched several sites, including the Ross Creek reservoir and two Dunedin properties - Ms Barclay's former Wakari property and another in Kaikorai Valley.
A former neighbour told the ODT one of his dogs - trained in scent and with ''a pretty good nose'' - used to sniff at a specific place in the hedge, prompting police to dig up a site at the neighbouring property.
It is one of a number of leads the police have investigated, including a tip-off from a gang prospect Ms Barclay, also known as Tania, may have been killed by a Christchurch-based gang.
Ms Kerr said there had been a suggestion her sister had been going to Christchurch to visit her and ''I would have opened up my arms to her if she had come''.
''She is like me. She knows a lot of people from all walks of life. We might know people from different gangs, but it doesn't mean we are gang-affiliated.''
Snr Sgt Inglis confirmed the Christchurch lead came to nothing, saying there was ''no evidence it was her and we did a full search on the pad, so that has been ruled out now''.
Police had talked to about 60 people - mainly family and friends - and some from her past days, which included prostitution, drug-taking, and several months behind bars.
''We are trying to establish more about her, her past, and who she associated with. Maybe someone is holding something back. There is [now an] incentive that they will ring us,'' he said.
Det Snr Sgt Inglis confirmed that there were three people of interest in connection with the case, and police had formally interviewed one.
He acknowledged that in the four years before her disappearance, Ms Barclay had worked hard to turn her life around.
That had been confirmed by family and friends.
Ms Barclay was a devoted mother to her three sons - one of whom was from her previous relationship while two were with Mr Brown, he said.
At the time of her disappearance, she was still breast-feeding her 5-month-old boy, taking children to school and appointments and was working part-time.
''Tania was always talking about her kids. She loved them ... I don't think she would have walked away from them,'' Ms Kerr said.
On Facebook, her 19-year-old son hit back at one commentator questioning the time it took his step-father to report his mother's disappearance.
''He may have reported her missing after four months, but why has it taken the cops 12 years to look into it?''
He told the ODT the ''whole investigation is a joke'', but declined to comment further.
Later he added ''... the main priority is how my mother's disappearance has affected the people that loved her to the present day''.
