A killer who sneaked a mobile phone into Castle Huntly was caught out when a plumber found it hidden behind a faulty radiator.
Drug dealer Andrew Gibson, who stabbed a teenager to death, had hollowed out a space behind the wall-mounted heater to hide the banned item.
The tradesman raised the alarm with prison authorities who spoke to the accused, eventually snaring him when his DNA was discovered on the phone.
In 2010, a jury cleared Gibson of murder but found him guilty of killing 17-year-old Connor Muir by stabbing him once in the chest during a fight in Livingston after rejecting his claim that he had been acting in self defence.
He was also convicted of two other knife attacks.
Solicitor James Smith, defending, said his client would be eligible for parole later this year despite being jailed for 13 years and six months in 2010.
He added: “He had worked well to get to the open estate and had been there about 15 months.
“As a result of this stupid decision he is back in closed conditions and the damage he has done has not been lost on him.”
Sheriff Mhairi MacTaggart said a custodial sentence was inevitable but told Gibson she did not want to interfere with his chance of an early release.
She jailed him for four months but made it concurrent to his current sentence.