Army bomb disposal experts had to deactivate a hand grenade after a woman wrapped the device in bubble wrap and took it to work.
The woman, who found the object while cleaning her mother's home, had planned to hand it in at a police station.
However, she left it in her car in a Stratford-upon-Avon car park and went to work. It was only when she told colleagues about her find that police were alerted.
"The hand grenade was found by the woman during a clearout of her elderly mother's home and it was her intention to hand it in to the police station." said a Warwickshire Police spokeswoman. "She went to work and mentioned to colleagues that she had the grenade in her car, and they told her to phone the police. This lady really shouldn't have moved it. She should have left it and called the police immediately."
As a precautionary measure, the car park, off Mansell Street, was closed off during the scare, on Monday, for the bomb disposal team to deal with the grenade.
The police spokeswoman added: "Obviously carrying hand grenades around is not something we would advise people to do but you would be surprised by the number of people who do move them. They assume it's safe because it hasn't gone off before, but you just never know.
"We called the Army straight away because they have the expertise to deal with it.
"We advise any member of the public who finds a hand grenade, or something that looks like a hand grenade or any device which may contain explosives, wherever it may be, to contact the police immediately. Do not move it - call the police, who will take appropriate advice from bomb disposal experts and deal with the device in situ."