A man made off with a cash box after threatening a security van driver with a knife outside an Aylesbury Tesco on Saturday morning.
While the ATM at the Jackson Road store was being refilled, the robber approached the driver.
He took the cash box and ran off into the Quarrendon estate.
The box was later found with no money taken.
Police were called at 6.12am and the area was sealed off while investigations took place.
DC Nicholas Hills said: “I would like to reassure the community in Aylesbury that robberies such as these are extremely rare and that Thames Valley Police is doing everything to catch the perpetrator.”
The driver of the security van did not suffer any injuries during the incident.
The offender is white, 5ft 8ins to 5ft 10ins tall, of slim build and in his late teens.
He was wearing a grey or light-coloured hooded top, with the hood up, a red and white scarf and dark bottoms.
Managers at Tesco said none of their staff who were on duty at the time saw the robbery.
One nearby resident said: “There were a lot of police walking around and police cars about.
“It was all going off.
“When I left at 9am the police were still there.
“I didn’t hear anything when it happened.”
Anyone with information should call police on 101.
Alternatively, if you wish to remain anonymous, called Crimestoppers on 0800 555 111.