Detective Inspector Anthony Cockerill, 42, was on the investigation team.
He has lived in Bridlington since he was 10 years old and is a former Headlands school pupil.
Ten years ago he was 32 and Officer in the Case working with Chief Inspector Bill Sparnon, senior investigating officer and Chief Inspector Tony Burke, deputy senior investigating officer. Both are no longer with the police force.
Today he leads the criminal investigation team dealing with all serious crime in C division, which includes Bridlington.
He said, as the missing person’s inquiry grew along with the mountain of information about Sybil: “It became apparent that the circumstances of her disappearance were not consistent with what we had been led to believe.”
Adding: “The investigation led us to arrest John Appelequist and he was subsequently charged with the murder of his wife despite the fact her body had not been, and still has not been, found.”
He described the trial, which was halted on its seventh day by Justice Andrew Smith in February 2004 who ruled the prosecution case would not be sound and directed the jury to return a not guilty verdict before hearing the defence evidence, as “disappointing”.
“The judge did accept the police had sufficient evidence to prove Sybil was no longer alive but insufficient to prove the murder charge,” he said.
“I have been on a large number of investigations of varying types and circumstances. “This was by far the most involved and intensive.
“The case is still very much open in our thoughts of finding Sybil.
“With advances in forensic science it would be possible to identify her. Her DNA is registered on the national missing persons database.
“Someone out there knows what happened to Sybil and where she is. If circumstances change and these people were able to tell us we would certainly welcome it.
“It is quite remarkable for her to have vanished without trace.
“It is extremely difficult for someone to do. Not to leave any clues, no doctors, hospital or dental visits, no benefits claims, no bank or financial transactions, - nothing,” said DI Cockerill.
The police search for Sybil was one of the largest and most intensive ever undertaken by Humberside Police.
By 2003 it involved around up to 30 detectives operating from an special incident room set up in Driffield.
Added to that were specialists including forensics, dog handlers, underwater search teams, even a botanist skilled at identifying if ground had been disturbed based on what was currently growing there.
The team had to put together thousands of pieces of the giant jigsaw which would give them the picture they needed of Sybil’s life in the hope that somewhere within it was a clue to what happened.
The detail was staggering, and for the most part unseen by press or public.
Her life and daily routines were minutely scrutinised, phone call patterns, supermarket visits, even what was on her shopping list, energy bill fluctuations at Cranbeck Close and much more.
For example, after March 2002 no telephone calls were made from the house between 8am and 4pm, the use of electricity had significantly dropped. But there were hundreds of other indicators that she was no longer at home.
“From analysis of all the data we had amassed about her everyday life we were able to deduce that she had ceased to exist much earlier than we had been informed.” said D I Cockerill.
The house, which had become a potential crime scene, was cleared and its contents taken away for forensic examination. There were no personal possessions, clothing, jewellery, photographs, even the car had gone, although the blue L registered Ford Fiesta was traced.
As police were to say at the time. “There was nothing in the house to suggest Sybil had ever been there.”
But the search went much wider.
The grounds of Marton Hall where John Appelquist had worked as a handyman, and part of the adjoining Sewerby Hall grounds and Links Golf club were examined on more than one occasion.
The Gypsey Race and other areas of water, pools and ponds were searched.
People she had worked with including her sister in law Pam Appelquist, John’s sister, and Sybil’s brother Anthony Hornby quickly came to believe she must be dead, and still do.
Every single discovery of anything that could have been human remains caused an alert and an investigation.
From bones in a lakes and waterways which usually turned out to be animal to the discovery anywhere in the country of an unidentified body or remains which could have been Sybil.
Today, 10 years on, Tony Cockerill is still notified of such finds.
However, none of it has been able to give even the smallest clue as to where Sybil is or what happened to her.
“I hope one day we will find her by chance or otherwise, not just for the family but for all the people who knew her and who worked so hard to find her.
“This was a very consuming investigation for me and other who worked on the case.
It is a Bridlington mystery we would rather see resolved”.
** If anyone who has not already spoken to police, has any information, however small about Sybil’s disappearance or where she or her remains may be DI Cockerill would like to hear from you.
He can be contacted via the 101 non emergency police number. alternatively contact any police officer. Information can also be given to Crimestoppers 0800 555 111.