When a 63-year-old man stopped along Upper Bukit Timah Road to refuel his lorry on Monday afternoon, he noticed another man leaving the petrol station on a black motorcycle with a masked licence plate.
Sensing that something was amiss, he tailed the rider to the Hillview area, stopping only when he thought the other man may have noticed him.
He then reported what he saw to the authorities.
The rider turned out to be a suspect in an armed robbery that took place at the Shell petrol station.
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The police were alerted to the incident at around 12.50pm that day.
Within four hours, the 48-year-old rider was arrested for allegedly robbing a petrol station employee at knifepoint - in what the police called a "brazen act".
Senior Assistant Commissioner Gerald Lim, commander of the Clementi Police Division, said: "The perpetrator came in, pointed a knife and threatened the staff at the petrol station. He managed to get away with more than $1,100 in cash."
The suspect was said to have ordered the 22-year-old employee to place cash from the register into a plastic bag.
When asked, a Shell spokesman said the employee will be commended for managing the situation well under pressure and ensuring the safety of her co-workers and customers.
The police said the suspect worked at a cleaning company.
Police officers worked with eyewitnesses and the petrol station's staff in reviewing closed-circuit television camera footage to eventually identify the motorcycle used.
"We subsequently found out where (the suspect's) haunts were, established ambush points, and managed to pick him up around Jurong East Street 21 on the same day, at about 4.50pm," added SAC Lim.
This was near the suspect's home.
Bank receipts, $499 in cash, and a motorcycle belonging to a member of the suspect's family were seized as evidence.
But the knife he allegedly used has not been recovered, as the suspect said he dropped it, according to the police.
This was the first time in 10 years that a cashier at a petrol kiosk had been robbed, said SAC Lim.
If found guilty, the suspect faces a jail term of up to 10 years, and 12 strokes of the cane.