Health care worker Maureen Gray helped a police artist to draw an auburn-haired man she saw in a common stair just hours before the teenager's naked body was found near her Greenock home.
She later told murder hunt detectives the pale man, who was breathing heavily and smelled of drink, made her feel "apprehensive".
With the help of statements made to police, Mrs Gray, 53, recalled the night in June 1986 at the High Court in Edinburgh.
She and boyfriend Joe - now her husband - had been to Greenock's Picture Hall to see a film, then to a Chinese carry-out in the town's Inverkip Road.
Mrs Gray said at the time she owned a flat in Nelson Street, just round the corner from Ardgowan Street where jeweller's assistant Elaine lived.
The couple went there to eat their carry-out, even though they did not live there, and stayed to watch late-night coverage of the Mexico World Cup on TV.
She told the trial that afterwards she went down the stair first, while Joe locked up, and saw a man standing there.
"I stopped briefly as I turned the corner. I did not expect him to be there.
"We just looked at each other and I went down the stairs."
Mrs Gray continued: "He looked quite agitated."
She told the trial: "When I passed he was breathing heavily. He was sweating and I thought he had been sick."
Mrs Gray described the man as "a mess" with his shirt hanging out and his hair wet.
She told her now husband she wasn't happy about the man being there and they sat, watching, in their car as he emerged from the stair door - looking up and down the street before he came out.
Mrs Gray said she also saw the man at the corner of Nelson Street, with his hands against a wall, peering down Ardgowan Street before he eventually made off in the direction of Anne Street.
John Docherty, 49, now of Dunoon, denies murder and claims that at the time Elaine, 16, was stripped and strangled, he was at home with his parents. The trial continues.