You probably wouldn't think twice if you happened to notice a few white stones outside your house or tucked underneath the wheels of your car - it's the sort of thing a child might easily do.
But a neighbourhood watch in Bristol is warning that you should be on your guard, as it could be a sign that your home is being scoped out by burglars.
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Residents of several roads in Portishead, Bristol, have reported finding the stones in recent weeks, and believe that they may have been left by a man delivering bags for charity donations.
He's described as white, slim and tall with receding long brown hair and a goatee beard. He was wearing a red T-shirt and driving a 'slightly scruffy' white Mercedes Sprinter van.
And Nick Gough, the area co-ordinator for Portishead's Neighbourhood Watch, says residents should alert the police if they have any concerns.
"There are reports of a male leaving white stones outside of various properties. These distinctive stones are believed to have been left outside of properties which look unoccupied," he tells the Bristol Post.
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In the past, tramps used to leave chalk marks outside homes to indicate, for example, houses where they were likely to be given a meal or little cash - and burglars are believed to have picked up on the technique to highlight homes that could be vulnerable.
So what are the signs to look out for - and what do they mean?
Stones under the wheel of a car are believed to be a way of checking whether it's been moved - which could indicate whether the home is actually occupied or not. If this is the case, then simply moving them could put the burglars off.
A chalked cross
According to police in Torbay, Devon, this mark indicates that a home is a good target. You could always just wipe it away - or, if you want to be clever, draw a circle around it, as this is believed to be a sign that there's nothing worth stealing.
Two overlapping squares is believed to mean that the inhabitant is nervous or afraid. Meanwhile, a mark that looks like an open book indicates a vulnerable female.
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Five circles or ovals in the shape of a flower is believed to mean that a home is wealthy.
The letter 'D'
A mark like a letter D - sometimes with a dot or line inside - is believed to mean that a property is too risky to burgle. If you find one of these, you may want to consider leaving it there...