When she returned home from work, Madam Tan Lye Lian saw her neighbour in handcuffs.
The cashier noticed that his eyes were red and that he looked devastated.
Madam Tan, 57, said: "When he saw me exit the lift, he said to me, 'Aunty, my child is dead'."
Her neighbour's four-year-old son had been found dead at their two-room rental flat in Block 149, Lorong 1, Toa Payoh, on Monday morning.
Police said no foul play was suspected.
The boy had been ill, neighbours said.
His father, 41, was arrested for drug offences.
The neighbours said the boy was cute and friendly.
"He would call me 'nenek' (grandma in Malay) whenever he saw me," said a neighbour, who wanted to be known only as Madam Chia.
"I saw him as my own grandson," added the 70-year-old, who lives just two doors away.
The boy, believed to be the fourth of five siblings, often had fainting spells and seizures, and had been in and out of hospital, the neighbours said.
The family, with two other boys and two girls, have been living there for almost 10 years. The youngest is said to be about two years old.
"They were a happy family who would say 'hi'. The kids even come by asking for biscuits and drinks sometimes," said Madam Tan.
"I just saw them go out as a family last Saturday," she added.
HINTS OF TROUBLE
But Madam Chia said there were hints of trouble sometimes.
She said: "I had heard them sometimes talking about getting a divorce."
Another neighbour, who did not want to be named, said the family had problems, but she declined to elaborate.
The police said they received a call at 7.18am requesting assistance.
When they arrived, the body of a boy was found lying motionless in the flat.
He was pronounced dead by paramedics.
Investigations are continuing.