What she was actually hearing was Canadian army supply clerk Denis Lortie spraying gunfire in the legislature in an attack that left three people dead and 13 others wounded.
Lortie's goal on the morning of May 8, 1984, was clear: "destroy the Parti Quebecois'' and then-premier Rene Levesque.
"I didn't know it was gunfire," Cannon said in an interview Thursday. "There was so much construction going on at the national assembly. You'd often hear drilling."
Cannon, who was 20 at the time, was working as a secretary for Pierre Paradis, who was, and still is, a Liberal member of the legislature.
As she began hearing on the radio what was unfolding, Cannon hid under the desk in Paradis' office.
"It was a Tuesday morning," she said. "There wasn't a lot going on at the assembly and I wondered if someone would think I was in my office and come and get me."
After 10 minutes, MNA Ghislain Maltais turned up.
"When we left the building, I only had one shoe. We'd been running. I didn't have my handbag. I had nothing."
Thirty years later, Cannon is still shaken when she sees archival footage from that day.
"I realize I was there," she said. "I was young and less aware of things. It could have been a lot worse than it was."
Some visible signs of the tragedy still exist, including a hole in a window shutter and a graze on a marble wall.
The legislature was minutes away from convening that misty morning when Lortie, dressed in camouflaged army fatigues, entered the historic building through a side door carrying two submachine-guns.
Witnesses said he shouted on his arrival: "Where are the MNAs (assembly members)? I want to kill them.''
Before arriving, he had delivered an audio tape to a Quebec City radio station vowing to "destroy the Parti Quebecois,'' describing it as "worse than the Communists.''
Georges Boyer, 59, and Camille Lepage, 54, both legislature messengers, were fatally hit by gunfire in the head as Lortie made his way to the legislative chamber.
A group of schoolchildren was pinned down in a room across from the chamber when Lortie entered the chamber and began spraying gunfire. Police later rescued the children, who escaped injury after a chaperon told them to get down.
Roger Lefrancois, 57, an employee of Quebec's chief electoral officer, was shot as Lortie entered the chamber. He died later in hospital.
Lortie installed himself in the Speaker's chair, firing wildly around the chamber. At one point, he howled in rage, pulled out his dentures and hurled them out into the room.
As wounded moaned from behind bullet-splintered benches, Rene Jalbert, the legislature's sergeant-at-arms, entered and tried to calm Lortie.
Jalbert, a retired major with the fabled Royal 22nd Regiment, spoke soldier-to-soldier to Lortie, offering him a cigarette and eventually getting him to release about a dozen hostages cowering in the chamber.
As one man limped past, Lortie said: "I'm sorry for wounding you, but that's life.''
Jalbert then took Lortie to his basement office where they continued talking because the sergeant-at-arms was worried about police moving throughout the building.
Once in Jalbert's office, the two men discussed the military and Lortie soon surrendered quietly to police.
Jalbert, who was later awarded the Cross of Valour, Canada's highest decoration for bravery, said later he never felt his life was in danger.
He said he did not know at the time that Lortie had killed anyone. Jalbert, who died in 1996, said when he got home and heard three people had died in the attack, "I had a couple of Scotches.''
Lortie was convicted of first-degree murder after his first trial in 1985 but a new trial was ordered because of errors by the judge. In 1987, he pleaded guilty to reduced charges of second-degree murder, allowing him to be eligible for parole after 10 years.
Francois Gendron, who was a PQ member of the national assembly in 1984, remembers the chaos and commotion outside the legislature that day.
"There were ambulances, police, security personnel going in and out," he said in an interview Thursday at a PQ caucus meeting in Scott, Que.
"It hit home because that's where we were heading."
Asked whether he believes such a tragedy could happen again, Gendron said the possibility has been greatly diminished because of increased security over the years.
"But anything can happen," he said. "You've seen what's happened elsewhere. We live in a difficult and modern world where there's a lot of questionable behaviour.
"It's fine and dandy to be more open in allowing people to have access to the legislature but at the same time there's the need for a minimum of protection and security for the men and women involved in public life."
And even though he wasn't actually in the national assembly at the time of the shootings, Gendron said the events still affected him.
"Yes, when you know there's a bullet hole in your desk and when you look at the clock and you realize that if you'd been in the room at the time there would have been a good chance you'd have been riddled with bullets."
A plaque in memory of those who died and were injured will be unveiled at the national assembly at a later date.
— With files from Martin Ouellet in Scott, Que.