SINGAPORE- He thought it was odd that the three men were withdrawing money again and again.
They were at an ATM just outside the Burlington Square building on Wednesday afternoon.
Holding a stack of cards, the men would withdraw cash, then go to the back of the queue so they could make another withdrawal.
Occasionally, they would walk over to an ATM less than 100m away at Sim Lim Square and do the same. This went on for two hours.
And it seems the 56-year-old security guard was not the only one who found their behaviour suspicious.
Someone else called the police.
The guard at Burlington Square, who wanted to be known only as Mr Noor, said of the incident: "They were acting very weirdly because they were trying to withdraw money again and again.
"On a few occasions, I noticed that they seemed to have trouble withdrawing the cash and would ask those behind them in the queue to go ahead first.