Gordon Darroch, 48, now a quality manager with an engineering firm, had gone for a walk on the night of the murder, in 1986.
In statements given to murder hunt detectives, Mr Darroch also said he had seen a man in his mid-20s dressed in black walking towards Greenock town centre down Ardgowan Street - where Elaine lived.
Mr Darroch was asked if the man he saw could have been Gavin McGuire, who was later jailed for the murder of schoolgirl Mhairi Julyan in Kilmarnopck. He replied: "It could potentially have been."
But advocate depute John Scullion, prosecuting, reminded Mr Darroch that in 1996 detectives had shown him a series of mug shots - which included Mr McGuire - to see if he could identify the man he saw around midnight in Ardgowan Street a decade earlier.
He had told the detectuives that none of the pictures had jumped out at him.
On trial at the High Court in Edinburgh is John Docherty, 49, who denies murdering jeweller's assistant Elaine on June 2 1986, and had produced a list of 41 possible suspects which, it is suggested, may include the real killer.
Docherty, of Dunoon, claims that at the time he is alleged to have stripped and strangled Elaine Doyle, 16, he was with his parents at their home in Anne Street.