The grandmother of missing four-year-old Gus Lamont has been fined $10,500 after admitting to a firearms offence unrelated to the boy’s disappearance eight months ago.

Josie Murray was fined $10,500 after a hearing in Adelaide magistrates court on Friday, where she pleaded guilty to one aggravated charge of possessing a sound moderator – a device better known as a gun silencer.

The 75-year-old was charged after a search of the family’s Oak Park station homestead, in South Australia’s far north, on 15 January.

Prosecutor Tania Stevens told the court the silencer was located inside a passcode-protected strongroom alongside several firearms, including one that was modified to fit it.

She said Murray was previously convicted in 2010 for falling to securely store firearms, after a fugitive trespassed on to the property and stole an unsecured firearm.

“That person then went on to commit serious offending with the firearm before taking his own life,” Stevens said.

Defence lawyer Andy Ey said two non-working firearms had been taken, while a third one was taken from Murray’s aunt’s property later that day. He said the trespasser used that third firearm to take his own life.

“That was an extremely traumatic incident for the entire family, with my client’s aunty being held hostage,” he said.

Ey also addressed what he described as “the elephant in the room”.

“It’s been an extremely difficult time for my client. A grandchild went missing on the property late last year,” he said.

“We see the media attention and number of police here – this has been a very, very difficult time for the family, trying to get through as best they can.”

He urged the magistrate Roderick Jensen to reject the prosecution’s call for a custodial sentence for the offence, which carried a maximum penalty of $75,000 or up to 10 years in jail.

“This isn’t some James Bond movie where you’ve had a villain or an assassin walking around the corners taking out targets with little more than a whisper of a gun – that is not what the court is dealing with,” Ey said.

Jensen imposed a fine of $10,500 and disqualified Murray from holding a firearms licence for five years.

Gus was last seen by one of his grandmothers playing at the family’s Oak Park station on 27 September.

On 5 February, South Australia police declared the boy’s disappearance a major crime and said someone living at the station was a suspect in the case and his likely death. But they stressed his parents were not suspects.

The following day, the grandmothers released a statement saying they had cooperated fully with the investigation and wanted nothing more than to find Gus and reunite him with his parents.

Gus’s disappearance sparked intensive searches spanning almost 500 sq km and involving hundreds of police and volunteers as well as aerial support and mounted units, which have failed to locate the boy or any items belonging to him.