Criminals casing homes to see if they worth burgling are thought to be writing secret signs outside the properties as reminders - or warnings.
Called the Da Pinci Code (or Da Pinchi Code) by police forces across Britain, the signs indicate what and who is in the property, whether it is worth raiding and when.
The codes are also sprayed on vehicles targeted by thieves, the Birmingham Mail reports.
Apparently an 'X' means the property is a good target whereas an X with a circle means there is nothing worth stealing.
If a property is too risky to raid a capital 'D' with a small line through the left of the letter is left.
Five circles means the owner of the property is wealthy whereas a symbol like an open book means a vulnerable female lives in the home.
West Mercia Police, which polices Shropshire, Herefordshire and Worcestershire, has previously said this is a myth and the symbols are in fact signs used by gas, electricity and other utility contractors to indicate what work needs to be done.
Other police forces have warned about burglars marking up property for potential theft.
And the alarm was raised in Staffordshire after two high performance cars were sprayed with Da Pinci Code symbols earlier this month.