Burglars are using coded chalk symbols to let other criminals know whether a property is worth targeting.
The marks, dubbed the Da Pinci Code, are made by would-be thieves to indicate if the house is vulnerable, revealing who is living in the house and whether there is anything worth stealing.
Chillingly one sign, in the shape of an open book, says a vulnerable female resident is the occupant and another symbol indicates a homeowner who is ‘nervous and afraid’.
The chalk markings were spotted on the side of a pensioner’s home in Walkden, Greater Manchester.
Police are now investigating the shapes after speaking to other residents in the area and have distributed leaflets in the area warning of the symbols.
The code has been previously been spotted in other parts of the country.
Resembling washing instructions with a series of crosses, circles and boxes, they have been found on walls and surfaces of homes as well as pavements and kerbs.
A simple 'X' means the home is a good target, while the same symbol outlined with a circle means there is nothing worth stealing in the property.
A capital D with a dash drawn in it indicates that burgling the house is too risky, while five circles in the shape of a star shows that a property is wealthy.
Other marks reveal if a house is alarmed or has already been burgled.
Salford police are now urging people who have noticed these symbols to take a picture and then wash them off the property as soon as possible. Police launched their investigation after receiving a report from an elderly woman on Sunday, August 25.
Chief inspector Sue Downey said: 'Police in Salford received a call from a woman in Walkden, who was concerned that a symbol had been marked on her house in chalk.
'Enquiries have been carried out and residents in the surrounding area have been spoken to, and we have received no further reports of any such activity.
'We always ask the public to remain vigilant and to report any suspicious activity to the police, which we will investigate.
'On this occasion we can find no link between the chalk symbol at this address and any criminality.'
In January, police in Torbay, Devon, posted the symbols on Twitter in a bid to warn homeowners that they may be a target for thieves.
In 2009, the affluent suburb of Tandridge in Surrey was also targeted by similar markings.