Properties are being marked with chalk in Biggleswade so the owners can be targeted by rogue traders.
Trading standards officers say potentially vulnerable residents are being singled out by the ploy.
Several chalk markings or crosses have been seen on homes in the Drove Road area of the town.
They have been removed with the permission of residents, according to the trading standards team.
It said: “We understand that these marks are used as a crude map by gangs of rogue traders.
“They then call at these homes with the intent of pressuring residents into having work carried out to their properties.
“The work is likely to be unnecessary and will be done to a very poor standard at highly inflated prices, well above what a genuine trader would charge.”
Officers are now investigating the issue, and say there is no cause for alarm.
A spokesman added: “But we would advise residents, particularly in the Biggleswade area, to be extra vigilant when they receive an unexpected visit from a trader.
“Please take a moment to check your own property and remove any unusual marks you may find.”
These marks are usually found on a boundary wall or on the pavement outside the property, so that rogue traders can easily see them from a vehicle.
The trading standards team says residents should avoid agreeing to any work following a random visit by a trader.
“Never feel intimidated into agreeing to any work if the trader says they are only in the area for a limited time, or claims the job is urgent.”