A large digital billboard outside a Liverpool shopping centre was apparently defaced by hackers on 29 May.
“We suggest you improve your security. Sincerely, your friendly neighbourhood hackers,” a message on the screen read.
It was posted on a large outdoor display at the Liverpool One shopping centre in the city.
Liverpool One said that it closed down the screen as soon as it was notified about the incident.
The message was tagged “#JFT96” – an apparent abbreviation of “Justice for the 96”, which is a reference to the 96 football fans who died in the 1989 Hillsborough disaster.
As well as a photo of the display posted to Reddit, another visitor to the shopping centre published a picture of the screen on Twitter.
“Our screens are operated by an external company [Elonex] which is currently investigating what happened as a matter of urgency,” a spokeswoman for Liverpool One said.
“As soon as the apparent hacked message appeared on the screen, Liverpool One immediately closed it down.”
The spokeswoman added that no other screens in the shopping centre were affected.
“We can confirm an incident occurred over the weekend on one of the 18 screens we operate at Liverpool One,” a spokesman for Elonex said.
“The incident appears to have been good-natured and not intended to cause offence or disruption, for which we are grateful.”
Several screens at the shopping centre had since been deactivated, according to cyber-security researcher Kevin Beaumont, whose added that his friend saw the message when it was posted on 29 May.
It appeared to be another example of how systems were often unknowingly left vulnerable to hackers, suggested cybersecurity expert Prof Alan Woodward at the University of Surrey.
“You might not think it matters, after all it’s just an advertising billboard, but who knows what else this system is linked to.”
“The next hacker might have darker intentions,” he told the BBC.