The last reported sighting of the 23-year-old was near St Enoch Subway station about 2.30am on Friday, but officers are checking claims he was seen about 30 minutes later at St Andrew's Cathedral, Clyde Street.
It is believed he climbed over a fence to get into the cathedral car park.
A spokesman for the Archdiocese of Glasgow said: "A man fitting the description of Michael was picked up on CCTV at about 3am entering the car park.
"He was seen climbing over a fence and then climbing back over it to get out.
"Police were called and officers arrived about three to five minutes after the man left.
"The footage has been passed to police."
It is understood Michael was then seen heading in the direction of High Street.
His father John, 52, brother Shaun, 20, and sisters Paula, 25, and Monica, 21, have been putting up "missing posters" across the city.
They have organised two searches to take place in the city centre today, meeting at St Enoch station, at midday and 7.30pm.
Mr Slaven, an estate agent said: "Anyone that wants to come along and help us look for Michael is more than welcome. We will be covering the area where we believe he was last seen, right up to High Street."
A police spokeswoman said: "We can confirm CCTV has been taken from St Andrew's Cathedral. Officers are reviewing this."
Michael, an electrician from Dumbarton, had been with friends before leaving Sugar Cube nightclub, Queen Street, alone.
As reported in yesterday's Evening Times, his parents said they were going through "living hell" as they waited for news.
Elaine Slaven, 53, said: "What we are going through is a total living hell. We have not slept, we have not eaten, we are just waiting.
"Not knowing is the worst. I would do anything to get my boy back."
Michael was spotted in CCTV images taken near St Enoch Station at about 2.30am.
According to his parents, he appears to have been acting erratically in the images.
Mrs Slaven said: "I think there is a very good chance Michael's drink was spiked.
"We have been told he was acting oddly in the CCTV images taken in St Enoch Square. Michael wouldn't take drugs. He is very against that sort of thing."
Just moments before being seen in St Enoch Square, the 23-year-old was spotted on CCTV in Subway, the sandwich shop.
The footage was recorded in the Queen Street branch, after Michael had left nearby Sugar Cube nightclub.
It shows Michael 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.
Michael has not used his mobile phone or accessed his bank account since he was last seen.
He is about 6ft, slim, fair, with short brown hair and brown eyes. He was clean shaven.
Inspector Steve Grimason, from Dumbarton police office, said: "Michael has been missing for several days now, and concerns are growing for his wellbeing.
"It is very unusual for him to go missing, and it is very unusual for him not to be in touch with his family or friends for such a length of time."
Inspector Grimason made a direct plea to the public, adding: "It is imperative we trace him because as each day passes we are becoming increasingly concerned for his safety."
Anyone with information is asked to contact Dumbarton police office on 101.