A pensioner, aged 90, had jewellery stolen from his house after being tricked by conmen in Chesterfield.
Derbyshire Police said a man knocked on the door of a house in Pewit Close, Holmewood, claiming to be involved in a local Neighbourhood Watch scheme.
He asked to check the rear of the property and invited the homeowner to go with him, which he did.
The victim later realised that his house had been broken into and that items of jewellery had been stolen.
Police are appealing for anyone who saw suspicious activity in the area to come forward.
The offence took place between 8.30pm and 8.40pm on Tuesday, May 10.
Officers also want people to be on their guard against cold callers who cannot provide identification.
A Derbyshire Police spokesman said: "Consider installing a door chain and make sure you keep your home secure.
"Do not open your door to anyone who calls without warning, no matter what excuses they give. Genuine callers will not call unannounced - they will arrange an appointment or give written notice about work taking place.
"Check the identification of any callers and, if they claim to represent a company or utility, call that company and check if they have cold callers in the area.
"Do not be tempted to leave your house with a caller, even if it is just for a minute and into your garden.
"If a cold caller is persistent or makes you feel concerned or uncomfortable, call police immediately."
Witnesses or anyone with information should call PC Stephen Sweet at Derbyshire Police on 101.