Homeowners in Spain have been warned to carefully inspect their front doors when returning from holiday.
It comes after police highlighted a simple trick burglars use to identify empty properties.
The Policia Nacional said residents should look for tiny pieces of plastic, paper or almost invisible threads placed between the door and its frame.
The objects can act as markers, or testigos, allowing thieves to determine whether somebody has entered a property while they have been watching it.
Burglars place the small marker in a discreet position and then return hours or days later.
If the plastic or thread is still exactly where they left it, it can indicate that the door has not been opened and that the property may still be empty.
If it has fallen, broken or disappeared, they know somebody has probably returned.
The Policia Nacional has repeatedly warned about the technique, telling residents to not touch suspicious markers and call 091 if they find them.
Police have previously identified several versions of the trick, including small transparent plastic strips wedged between the door and frame, pieces of paper inside locks and extremely thin lines of glue or silicone.
A Policia Nacional spokesman explained that some criminals now place very fine threads around the lock itself.
A resident can insert their key without necessarily noticing it, but when the criminals return they can see whether the marker has been disturbed.
The method has also been documented during investigations into organised burglary gangs operating in Spain, which have marked potential targets before returning several days later to see which properties appear unoccupied.
Police in Murcia have additionally warned of an increasingly discreet version known as the ‘invisible thread’ technique, where glue or silicone is used to create a very fine connection between the door and frame, often at the top or bottom where residents are less likely to notice it.
The latest advice is particularly relevant at the end of the summer holiday period, when thousands of people are returning to homes that may have been empty for days or weeks.
