Michael Slaven, 23, from Dumbarton, was last seen in the early hours of Friday.
Officers are investigating claims Michael was last seen at St Andrew's Cathedral, in Clyde Street, at 3am on Friday.
Michael, an electrician, had been with friends before leaving Sugar Cube nightclub, in Queen Street, alone.
It is believed he climbed over a fence to get into the car park at St Andrew's Cathedral .
An Archdiocese of Glasgow spokesman said: "A man fitting the description of Michael was picked up on CCTV at around 3am on Friday, entering the Cathedral car park.
"He was seen climbing over a fence to get into the car park and then climbing back over it again to get back out.
"The police were called and officers arrived around three to five minutes after the man left. The footage has now been passed to the police."
It is understood Michael was then seen heading in the direction of High Street.
His family, including father John, brother Shaun, 20 and sisters Paula, 25 and Monica, 21, have been putting up missing person posters across the city.
They have organised two searches to take place in the city centre today, meeting at St Enoch underground station, at midday and 7.30pm.
Mr Slaven, 52, an estate agent said: "Anyone who wants to come along and help us look for Michael is more than welcome.
"We will be meeting at St Enoch underground station and covering the area where we believe he was last seen, right up to High Street."
A Police Scotland spokeswoman confirmed that officers are investigating the possible new sighting.
She said: "We can confirm that CCTV has been taken from St Andrew's Cathedral.
"Officers are currently reviewing this."
The Herald revealed yesterday that Michael's parents, John and Elaine, 53, were going through a "living hell" as they desperately waited for news.
Michael was spotted in CCTV images taken near St Enoch Subway station at around 2.30am.
According to his parents, he appeared to have been acting erratically in the images, raising fears that his drink may have been spiked.
Mrs Slaven said: "I think there is a very good chance Michael's drink was spiked.
"Michael wouldn't take drugs. He is very against that sort of thing."
The 23-year-old was also spotted on CCTV in the Subway sandwich shop, in Queen Street.
Michael is seen choosing his food and a bottle of water, before paying, and calmly walking out.
He was wearing a white t-shirt with a grey square emblem on the chest, olive green chino-type trousers and dark plimsolls.
Mr Slaven obtained the footage from a camera at the sandwich shop, where Michael was spotted at around 2.25am.
Michael has not used his mobile phone or accessed his bank since he was last seen.
He is described as around 6ft, slim, fair and is clean shaven with short brown hair and brown eyes.
Inspector Steve Grimason, from Dumbarton Police Office, said: "Michael has been missing for several days now, and concerns are growing for his wellbeing."
Anyone with information is asked to contact Dumbarton Police Office on 101.