Roger Kearney, 67, was jailed for life with a minimum of 15 years in summer 2010 for the murder of 40-year-old Paula Poolton.
Mrs Poolton, from Titchfield, was found stabbed to death in the boot of her car near Swanwick railway station in 2008.
Campaigners have long been investigating the case, and found some evidence had been ‘lost, contaminated or destroyed’ by Hampshire police.
The force has previously said the officer in the case destroyed items without consulting the senior investigating officer.
Now charity Inside Justice has said ‘people close’ to the victim said Mrs Poolton may have been in a ‘relationship or friendship’ with another man close to her death.
The charity did not provide further details.
Louise Shorter, chief executive and investigator at Inside Justice said: ‘There has been further speculation of this on social media and if true, could be highly relevant to the case.
‘We need to know one way or the other, so I am appealing to anyone who has information about a relationship or friendship to come forward, speaking in confidence if need be.’
In 2016 the BBC screened Conviction: Murder at the Station showing Ms Shorter investigating the case and finding tapings from the victim’s clothing and a carrier bag had been lost, destroyed or contaminated.
There is guidance in place to stop this from happening but 80 per cent of police forces are unaware, the charity claimed.
Postal worker Kearney, who was Mrs Poolton’s lover in an affair and who knew her while working at Southampton FC together as stewards, denied murder.
Months after being jailed he even launched a bid to overturn his sacking from Royal Mail, saying it was unfair dismissal over his murder conviction.
Mrs Poolton’s parents told The News of their anguish in 2016 at the BBC documentary.
Her mother said: ‘He’s not innocent. Twelve jurors all agreed he was guilty.’
In a new video on the case, Kearney’s daughter said: ‘He’s not a callous murderer.’
Hampshire police response
A Hampshire police spokeswoman said: ‘Hampshire Constabulary has co-operated fully with the Criminal Cases Review Commission (CCRC) and all records relating to our investigation into the murder of Paula Poolton have been shared with them upon request.
’We have confirmed to the CCRC that some forensic exhibits have been disposed of since Roger Kearney’s unsuccessful applications to appeal. These items did not become actual court exhibits during Roger Kearney’s trial and were not used as part of the criminal proceedings or the evidence that led to his conviction for Paula’s murder.
’The exhibits and disclosure officer in this case disposed of the exhibits because they had either been forensically tested to the point of destruction, were deemed a health hazard or had not become actual court exhibits and because Roger Kearney had exhausted all legal avenues to appeal his conviction.
‘These exhibits were available to the court at the time of Roger Kearney’s trial and were not used. Their subsequent destruction has no bearing on the investigation into Paula’s murder or Roger Kearney’s conviction in 2010.
’As a force we constantly seek to ensure we uphold best practice and although we don’t believe the destruction of these exhibits has had a detrimental effect on this case, we acknowledge that the senior investigating officer was not consulted before their destruction.
’We have reviewed our management of forensic exhibits and changed our policy and working practices to make sure the retention and destruction of forensic exhibits is appropriately managed.’