But DS Fulcher - who has always insisted Halliwell has more victims - believes the stash could be a sick memento of the killer's undiscovered crimes, reflecting a roll call of the women he has targeted.
The revelation comes in an explosive new book, Catching A Serial Killer, which Fulcher began writing when he quit Wiltshire Police for failing to adhere to police protocol during the investigation.
The Sun reports him as saying: "At the bottom of a pond that was 8ft deep they found O'Callaghan's distinctive boot. Yet that wasn't all. Halliwell had been busy.
"Buried around the pond were more items of women's clothing. I wondered, did they belong to the six other victims I suspected Halliwell of killing? But I had my maths wrong.
"Around the pond were not six other items of women's clothing. There were 60.
"Halliwell might have been far more prolific than even I had feared."
He adds: "Are you seriously telling me that on two occasions he brutally murdered two women - and then there's nothing before or since?"
Police began investigating Halliwell when O'Callaghan was abducted on her way home from a nightclub in Swindon in 2011.
CCTV taken near the Suju nightclub showed her getting into a taxi which the father-of-three owned.
Police then kept watch on Halliwell for two days - hoping he would inadvertently lead them to O'Callaghan's body - but arrested him on suspicion of kidnap five days later when they feared he would kill himself.
Aware of his suicidal tendencies, DS Fulcher arranged an interview team to conduct a "Safety Interview", under caution, at the scene of his arrest.
But when Halliwell kept answering "no comment", DC Fulcher authorised officers to bring Halliwell to the nearby Iron Age fort of Barbary Castle - where they believed O'Callaghan was buried - for an "urgent interview".
Halliwell eventually led DS Fulcher and his team to an isolated lane, where he said O'Callaghan's body was lying in the open.
Then, as he was having a cigarette at the scene, Halliwell confessed to "another one", telling DS Fulcher: "I need help, I'm a sick f*****."
The father-of-three then led the detective to the spot in rural Gloucestershire where he had buried Godden in a shallow grave after strangling her in 2003.
At that time, police had no idea she was missing.
DS Fulcher was later nominated for a Queen's Police Medal, the highest honour in policing, for his work in extracting the brutal killer's confession.
But, in a cruel twist, he found himself fighting for his job after breaking police guidelines by taking Halliwell to the site, rather than the police station.
Fulcher was disciplined for misconduct and later resigned, losing his £500,000 (NZ$873,363) police pension and selling his house.
Haliwell was jailed for life in 2012 for beating and stabbing O'Callaghan - who he believed resembled his mother - to death, before pushing her body into the 12ft ravine.
But a judge ruled his confession about Godden inadmissible because DS Fulcher had repeatedly failed to correctly caution the killer and questioned him without a lawyer.
It took four more years of campaigning for it to be allowed.
Other officers then had to find more evidence and, thanks to unearthing the stash and other circumstantial and forensic evidence, Halliwell was convicted of killing Godden and given a whole-life jail tariff.
During the trial, Halliwell gloated about how he had ruined DS Fulcher's career, telling him: "By the way, it was a pleasure ruining your career, you corrupt b*****d."
As he passed sentence on Halliwell, Sir John Griffiths Williams told the killer: "But for your confession, I have no doubt Becky's remains would never have been found.
"You then tried to manipulate the police and court process to try to avoid getting what you deserved."
At the time of his sentence, police said they were "very, very clear" that he had more victims.
Speaking after, Detective Superintendent Sean Memory said: "I can't believe that Becky was his first offence, from being a burglar in the 1980s to a murderer in 2003. There was a significant gap in his offending behaviour.
"On top of that, Sian wasn't murdered until 2011 so what happened in the interim eight years?"
Halliwell had already been linked to several other killings and disappearances over the past few decades.
DS Fulcher said he had always been convinced Halliwell had targeted more women, while it also emerged that the former butcher also once asked a fellow prisoner how many women a person had to murder to become a serial killer.
One of the cases to which Halliwell has been linked is that of mother-of-four Linda Razzell, 41, who disappeared from Highworth, near Swindon, on March 19, 2002.
Her husband Glyn has spent 13 years in jail for her murder but maintains his innocence, with his lawyer saying he would be "very surprised" if he had committed the crime.
It has since emerged Halliwell was obsessed with Linda after doing building work at the Razzells' home four years before she died.
Crucially, Razzell went missing on March 19 - a day considered potentially 'special' to Halliwell and the same date O'Callaghan went missing.
The date became pertinent to Halliwell's case because it was when he was dumped by his girlfriend while he was in prison for previous crimes.
Describing how the crime is "fitting" with Halliwell's pattern of behaviour, DS Fulcher has previously urged officers to look into possible links between Halliwell and Razzell's case.
Detectives said there was a strong suggestion Razzell had been abducted after her mobile phone was found near her abandoned car.
Officers have since said that Razzell and Halliwell had a "direct relationship".
Meanwhile, Halliwell's father lived a few streets from the home of chef Claudia Lawrence, who has been missing since March 19, 2009 - the same day as Halliwell killed O'Callaghan.
Lawrence's father Peter has previously urged police to look at any links between the double murderer and his missing daughter, although North Yorkshire Police have denied the link.
Referring to a potential link, DS Fulcher said: "It fits his pattern of behaviour - abducting women walking alone either late at night or early in the morning.'"
Another potential victim is prostitute Sally Ann John, 24, who vanished from Swindon in 1995. She and Halliwell lived on the same street.
The last sighting of her was in the Manchester Road area, in the town's red-light district, at 10.45pm on September 8, 1995. No trace of her has ever been found.
Godden's mother Karen Edwards has previously claimed that Wiltshire Police failed to act on evidence that she has given them about the three disappearances.
She said: "He is definitely a serial killer. I believe he has been up and down the country murdering young women.
"Serial killers are usually triggered by dates. That was the day that Halliwell broke up with one of his partners.
"Halliwell was familiar with York - his father lived in Huddersfield - and the description of Claudia's murderer is identical to him - a left-handed smoker, 5ft 8-10in, with slightly receding hair and a skinny build.
"Claudia was reported missing from her home on March 19, 2009 - exactly two years to the day before Sian. It is also the same date that a woman called Linda Razzell disappeared."
Another potential victim is Sally Brewin, who disappeared from her parents' home in 1994 just months after she became penfriends with a man who was in prison in Oxfordshire.
The whereabouts of Vietnamese immigrant Thi Hai Nguyen, 20, has also been unknown since she went missing from her temporary home in Swindon in June 2005.
The disappearance of Tina Pryer, 39, has also been linked to the killer.
Earlier this year, police searched the Swindon garden of the double-murderer for more bodies, part of an ongoing investigation carried out by the Brunel Major Crime Investigation Team.
Wiltshire Police have since issued a statement saying there is no evidence linking him to any other murder in this country.
But Fulcher - who now works training police in war-torn Somalia - added: "There is only no evidence because they haven't spoken to the witness."
