Ms Brenda Tan let her son out of her sight - and in a few seconds he was nowhere to be found near their Hougang home.
It took a nail-biting three hours or so before she received a call and learnt the boy, who has autism, had been found - at Changi Airport.
Last Friday, Ms Tan was about to leave her home at 7.30am with her son Calder Kam, 12, to wait for the school bus to take him to St Andrew's Autism School.
He stepped out of their flat, but she went to get a book for him to read in case the bus was late. When she returned, he was not there.
She went to the void deck, then checked each of the 18 floors in her block. She contacted a woman on the school bus, but learnt he had not boarded it.
FIND CONTACT PARTICULARS
It's important for the public to help: When they come across these people who often cannot answer questions well, find contact particulars they (children with autism) may have with them and contact the caregivers.
MS BRENDA TAN, mother of Calder, a 12-year-old boy with autism.
She left her contact with a fruit stall vendor, a porridge stall owner and a neighbour. She also related her situation through a Facebook post which was shared more than 1,000 times. Her husband lodged a police report.
Ms Tan, a 42-year-old writer, told The Straits Times yesterday: "My son was lost twice before. Once, he wandered off when I was buying groceries at Hougang Green mall. Another time, he ran down the stairs and across the corridor to another part of the block."
Each time, she had prayed, and "somehow walked towards the right direction until I saw him".
But the latest incident was different. "This is the first time he took public transport to somewhere so far that we couldn't find him ourselves," said Ms Tan, who also has a nine-year-old daughter.
Ms Tan believes Calder walked to a nearby bus stop, then took service 27. The ride to the airport takes about an hour, so he could have been there for at least two hours before auxiliary police found him in the arrival area at Terminal One, she said. They found Ms Tan's name card - which has the line "This belongs to my son Calder who has autism" written on the back - in his bag and called her at around 11am.
"The name card saves the day," she wrote later, in a blog post.
But she is considering getting a tracking device, after some parents of special needs children suggested she do so. Such devices can be attached to an object which is located via a mobile app.
Ms Tan said it is not unusual for children with autism to get lost.
"Many cannot express themselves... so they just walk and walk while caregivers scout for them.
"It's important for the public to help: When they come across these people who often cannot answer questions well, find contact particulars they (children with autism) may have with them and contact the caregivers."