Barnes said he was returning to his cell after a visit when an officer stopped him and said: "I've got some news for you: You're going to be called for a urine test in the morning."
Intuition born of decades in state prisons clicked: It was a prerequisite to parole, Barnes realized.
Though the parole board twice rejected him in 2011, Barnes said, "In my mind's eye I thought, well, maybe the slip is coming back approved."
At noon Friday, he walked out of Graterford and was driven away by the same friend who visited the day before.
"I knew I was innocent," he said. "Having said that, I also accept the responsibility of shooting Officer Barclay. It's the most shameful act I've ever done. And I live with that, what it did to his family and what it did to mine. How it altered his life. I'm deeply ashamed."
Prisoner advocates, editorial writers, and others have railed about keeping Barnes - who has heart problems, high-blood pressure, and macular degeneration - in prison long after May 24, 2010, when a Common Pleas Court jury acquitted him of murder in Barclay's 2007 death.
"My case is unique; it doesn't happen every day," Barnes said. "But I see fairness in the criminal justice system. . . . Negative behavior brings on negative results. I can live with that."
Barnes was already paroled once, in 2006, after 16 years of a 20-year sentence for shooting and wounding Barclay, a 23-year-old rookie, during a 1966 burglary of an East Oak Lane beauty salon.
The shooting turned Barclay into a paraplegic, and he lived a purgatory of a life before dying of infection at 64.
After Barclay died, the District Attorney's Office ordered Barnes arrested for murder. Authorities found that Barnes, who was working at a Roxborough supermarket, had a cellphone and car keys - parole violations - and he went back to prison.
City prosecutors argued that Barclay's death was a direct result of the 1966 shooting, and experts from the city Medical Examiner's Office testified that the fatal bladder infection was a common result of the paralysis that would not have afflicted Barclay had Barnes not shot him.
Barnes refused a plea offer. "I gave him a miserable life," Barnes said, but "I'm not a murderer."
Barnes' pro bono defense attorneys, Samuel W. Silver and Bruce P. Merenstein, both of the Schnader firm, argued that Barclay had sustained too many intervening injuries to trace an unbroken line from shooting to death.
Three car accidents and two wheelchair accidents, Silver argued, accelerated Barclay's deterioration, as did physical abuse from 2001 to 2003 when he had live-in caretakers in his Levittown home.
Hospital records noted that Barclay complained that the boarders kept him confined in his room, underfed him to the point that he developed scurvy, and left him unwashed and sitting in his own waste. He contracted incurable bedsores, went into a nursing home, and died four years later.
But acquittal did not result in freedom and Silver and his team filed a federal suit contending that the parole board was violating Barnes' due-process rights.
In February, a federal magistrate judge recommended that Barnes be released at once.
Barnes' parole angered many in law enforcement, including Lodge 5 of the Fraternal Order of Police, whose members packed his trial two years ago.
Nor do they buy Barnes' prison epiphany: He said he turned his life around after realizing his career as a not-too-successful criminal would end in death in prison.
"It's a lot easier to blame me," said Barnes, who says he has "no animosity to anyone. I've never been more happy because I turned my life around."
Though paroled, Barnes, who lives in the Philadelphia area, remains under supervision, possibly through 2030. A violation of the law or his parole rules - no alcohol, for example - would put him back to prison.
It's not going to happen, he said.
He wants to get a job "if only to keep me busy." He wants to resume the community service he began during his 2006 parole: counseling young inmates and speaking in public about the price of breaking the law.
What he cannot do under parole rules is apologize to Barclay's relatives.
It's something that conflicts him to this day.
"Somebody gets shot, how do you go up to them? Is an apology enough? You know it really isn't. I didn't have the courage to face them. I'd express my sorrow to other people, but I never followed through with it."
Contact Joseph A. Slobodzian at 215-854-2985 or email@example.com, or follow on Twitter @joeslobo.