Police in Hampshire were called in to guard an unlocked high street bank.
Officers were called to the HSBC branch in Tadley on Saturday after a member of the public arrived to find the door open, but no staff inside.
Police said everything inside the branch appeared to be in order and nothing had been taken. They guarded the bank until the keyholder arrived.
A spokesman for HSBC said it was not clear whether the bank had been left open all night.
He said: "We are still investigating the chain of events.
"It's hard to say how long the door was open - but unlikely it was open all night.
"Nothing untoward happened when the door was open.
"We would like to thank the member of the public who reported the open door."
A spokesman for Hampshire Police said officers arrived shortly before 1000 BST on Saturday and stayed while the keyholder was contacted.
He said: "There was nothing suspicious about it apart from the fact that the door was open."