A security guard is alleged to have vanished with 40 cash boxes worth £1 million ($1.3 million) from his abandoned van.
Dad-of-three Joel March, 36, left his armoured G4S van on a double yellow line in a quiet road in Clapham, London, just before 9am on 23 April.
But residents became suspicious and alerted security company G4S, and he was later arrested in nearby Brixton.
Speaking to The Sun, a source explained how he had thought something was up when they spotted the van in a residential area, saying there were no shops in the direct vicinity.
Having peered in, he said: "I saw some strips of paper with 'mum', 'dad' and phone numbers. That worried me in case someone had come to harm."
The resident called G4S at around 5pm to raise the issue with them, but said they didn't seem 'particularly concerned'.
The Met Police then arrived at 7pm, responding to 'reports of a theft of cash deposit boxes'.
The resident added: "Some of the guard's family turned up at around 8pm very upset. They said he'd been under a lot of pressure."
Before March's arrest, his mother - who shares a home with March and his kids - said he had left his personal phone at home on the day of the incident.
She told the Sun: "He was a bit stressed before this but I can't think why he would do it. As far as I know he never had any money issues."
A Scotland Yard spokeswoman said: "Police were called at approximately 7pm on Tuesday, 23 April to reports of a theft of cash deposit boxes from a cash-in-transit vehicle.
"Officers attended a location in the Larkhall Rise area of Clapham where the vehicle was parked. There was no sign of forced entry to the vehicle.
"An investigation was launched by the Met's Flying Squad and a 36-year-old man was arrested by officers on suspicion of theft at approximately 3pm on Wednesday, 24 April in SW2.
"He has been taken to a south London police station. A quantity of cash has been recovered."
Ex-Met-chief superintendent Barry Phillips added: "Something has gone seriously wrong.
"The van's movements should have been monitored and the controllers should have been in regular communication with the guard."
A G4S spokeswoman said the company has now launched an investigation into the incident.
The spokeswoman said: "These incidents are extremely rare in our UK Cash business and we are working closely with the Metropolitan Police to resolve the matter. The financial impact is not material."
The Met Police said enquiries are ongoing.
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