MADELEINE McCann’s abductors may have daubed coded messages on the outside walls of the family’s holiday apartment to indicate when she was alone. 

Police in Portugal’s Algarve recently discovered arrows or circles are being used by criminals to signal a chance to break into homes. 

A horizontal arrow means property is easy to get into during the afternoon. Semi-circles underneath a line indicate a child is alone in the evening.

The signs, believed to originate with Brazilian crime gangs, have been found on or near properties across the popular holiday region. 

Madeleine McCann was three when she vanished in Praia da Luz four years ago as parents Kate and Gerry dined nearby. Police suspect the apartment may have been staked out for days. 

And witnesses saw a man watching it in the week before Madeleine’s disappearance in May 2007.   

Dave Edgar, a former detective now investigating the case for the McCanns, has said: “Someone just doesn’t go in, a passer-by, and pick up a child and take it.   

“These things are planned. They may even have been watching the apartment for a week or more.” 

Police in Portugal hope publicising details of the codes may help prevent crime in the region.

A police spokesman said: “The signs indicate to other burglars how easy the property

is to break into, as well as indicating when the house is empty or more likely to be empty, if there are elderly people or children alone in the house, among others.”

And he warned residents: “If you see any of these signs around your house, you should remove them immediately and contact your local police.” 

Clarence Mitchell, spokesman for the McCanns, said anything the police were doing to raise awareness of possible crimes in the area was welcomed.  

Sales from Kate McCann’s book Madeleine, released in May, are believed to have now passed 200,000.  

And Kate, 44, and Gerry, 43, from Rothley, Leicestershire, hope new information on their daughter’s case will emerge.  

A Metropolitan Police team of detectives is reviewing the evidence in her case to see if any clues have been missed.