But on this occasion her colleague lay in wait for the 51-year-old supervisor.
They had butted heads in the preceding weeks.
Merritt had taken to parking his car in a disabled spot while cleaning at Otago Polytechnic and fellow employee Des Hurring said he ''flew off the handle'' when Ross bailed him up about lacklustre cleaning of windows.
The previous evening - only several hours before the victim was preparing to knock off - she gave the 21-year-old defendant a disciplinary letter detailing his perceived shortcomings, which would require a meeting with company bosses. In it, critically, was the threat of dismissal.
Although Spotless regional operations manager Craig Soal told the court neither offence was serious enough to have Merritt sacked, the employee did not know this at the time.
Without his job, no money would be coming in to pay for the rebuild of his beloved Morris Minor, which he had thrown himself into with his father.
''She became the focus of his anger,'' Crown prosecutor Robin Bates said.
Consumed by rage, Merritt armed himself with a hammer, dressed himself in a dark size-14 woman's hoodie he knew no one could connect to him and drove from his Kaikorai home back to work. He knew Ross would finish her shift before 1am.
During the two weeks he was on trial before the High Court at Dunedin, the Crown could not provide direct evidence of where the killer waited, for how long or how the attack progressed.
The physical injuries to the victim spoke volumes, though: 14 blunt-force lacerations to her scalp and 32 defensive injuries to her arms and hands.
But it was the injuries to Merritt that sealed the case for the Crown.
Ross defended herself against the young assailant, flailing as he rained down blows on her with the hammer. The scratches gouged into his face and arms were inexplicable other than to finger him as the murderer.
Forensic testing of her fingernails also turned up DNA 430 times more likely to be from Merritt than anyone else.
He told his parents, colleagues and the police he had scratched himself in his sleep.
The court heard Merritt had eczema, which the defence put forward in a bid to give some validity to his story.
In an interview with police on December 4, he told Detective Graeme Smaill it had happened before.
''Last time, I noticed I had scratches on my back one time. I don't know how it happened. I presumed it happened in my sleep like this one. I just thought it was weird,'' Merritt told the officer.
The detective said: ''You've got short nails. I'll just give you a second to maybe rethink that.''
But Merritt would not budge.
''I went to bed, didn't have any [scratches]; woke up, had some.''
Detective Smaill left the room to let the defendant dwell on his unconvincing response and came back five minutes later.
''You don't want to tell me the whole truth at the moment?'' he asked.
''No,'' Merritt told him.
Merritt showed no concern for the victim. When he showed up for work the day he killed Ross, staff were reeling at the news.
His colleague, Aiden Kelsall, asked the defendant how he felt about their boss' death.
''I don't care. You know how I felt about her,'' Merritt said.
It was not the first time he had voiced his hatred for Ross, according to Kelsall.
After being publicly reprimanded, Merritt said he wanted to burn the woman's family alive in front of her.
''The lazy bitch. It would be fine if she died,'' he said several times, the court heard.
When given a chance to retract that when taken into the police station, Merritt refused.
''Sounds like something I'd say,'' he told Smaill.
And Merritt unflinchingly admitted he did not care Ross was dead.
''A woman's just lost her life and you don't care she's dead, because she spoke to you rudely?''
''Yeah.''
Further to evidence about their deteriorating relationship, the prosecution put forward a substantial amount of forensic evidence.
The bloody murder scene did little to help the Crown peg the killer, though it portrayed the brutality of the attack.
''Spines of blood'' radiating out from the victim's head suggested she had been smashed with the hammer while she lay face down in the car park, Bates said.
A bloody hand print on a van door showed the killer had pulled Ross from the driver's seat as she tried to escape.
But no DNA testing was done on anything at the scene.
On December 4, police searched the Merritt family home at 4 Nairn St. Over four days they combed every room and the outside of the property.
Blood found on the bathroom tap and vanity, which was classed as extremely likely to be from Ross, was a significant discovery but the breakthrough came on the final day of the search.
Officers opened the yellow-lidded council wheelie bin at the back of the house to find three pizza boxes, plastic bottles and old newspapers.
Under them was a dark item of clothing.
''It was immediately obvious there was some sort of small, rubber-handled tool'' poking out of a pocket, Constable Karl O'Dowda told the court.
Later inspection turned up a glove in another pocket with ''A'' embroidered on the cuff.
Each member of the Merritt family denied they were familiar with the items but the coincidence between the stitched capital letter and the defendant's first name did not escape Bates.
''You might think it's something a mother might do to identify a set of gloves,'' he told the jury.
More convincing was what was found on the blood-covered items. Forensic testing showed cells on them were one million million times more likely to be Ross' DNA than anyone else's.
The slightly bent tack hammer provided courtroom theatre, but the invisible evidence on the hoodie tied Merritt to the killing.
A swab showed another DNA profile was also present on the garment, which was 500,000 million times more likely to belong to him than anyone else.
The defence argued it could have been transferred from anything in the wheelie bin that had Merritt's sweat, saliva or skin cells on it.
The bin had sat there for 62½ hours before the police secured the area, Stevens said. Anyone could have dumped the items there.
But the coincidences were too much. For the defence's story to have credibility it needed someone with a reason to kill Ross and frame the defendant.
There was no one.
Merritt told police he got home just after 9.30pm on December 1 and was in bed between midnight and 1am. He got up at 9.30am the next day, he said.
But his cellphone told a different story.
At 1.34am, when the killer was probably heading home after the murder, Merritt's phone pinged off the Dunedin North cell tower on the corner of Hanover and Great King Sts.
The Crown called Spark expert Lawrence Watson to explain the significance of the data.
As part of his investigation, Watson drove three routes from Strathallan St to Nairn St while monitoring two phones' activities. He concluded the only way Merritt's phone could have connected with the Dunedin North cell site was if it was on Highgate or on the city side of that road.
Radio waves, Watson explained, could not bend. For Merritt's phone to use that tower from his house, the waves would need to bend almost 90 degrees.
Over a gruelling nine days of evidence, Ross' family sat at the back of the court as they heard lawyers and witnesses pick their way through the details of her death.
After repeatedly reliving the ordeal, the victim's partner, Richard Leckie, said it was ''perfectly obvious'' what had happened.
He did not want to go on record with a gushing tribute to Ross, saying simply: ''The people that need to know, know.''
''Nobody will ever forget Karin. Dunedin will never forget Karin.''
