The plaque reads “Ian met Yvonne Here on 5th Oct ‘67 and fell in love” and is on a billboard at the Institute in Aberdeen.
It was discovered by staff a few weeks ago and they are now keen to track down the people involved.
A social media media campaign has now been launched in the hope that someone who knows the couple will come forward.
Nightclub sales executive Meredith Robinson said a social media user has already been in touch over the weekend to let them know of a similar plaque in a city park.
The second one reads: “Ian courted Yvonne here 1967-68. Had lots of fun”.
Ms Robinson said: “Nobody really knows how long the plaque has been there for.
“Someone actually got in touch on Facebook to say there was another plaque in Victoria Park.
“It’s getting quite exciting so we’re on a hunt for find out who they are and find out about their story.”
Ms Robinson said the plaque was on a poster frame near the front door but staff did not come across it until a few weeks ago.
It is not known when the billboards were erected on the wall of the building which used to be a music venue and was also once a cinema.
She said: “We are hoping there’s a nice story behind it.
“It would be good to ask them if this is where they first met and what they did when they came here.
“We want to find out their story. If they put the plaque here the building obviously means a lot to them.
“It would be nice to find out what they are up to and if they are still together.”
Nightclub staff now hope that family or friends will recognise the names of the couple and come forward.
The online search has already reached 12,000 people since being launched at the end of last week.