Criminal gangs marking houses worth robbing

It’s reported criminal gangs are leaving chalk marks outside homes, in an effort to alert other burglars as to whether this property is worth robbing.
The Irish Examiner says community alert groups have reported seeing such codes left outside homes in Dublin, Drogheda and Limerick.
Its understood there are 8 symbols, and they range in meaning from things like like  this home is a good target, vulnerable female easily conned  to  alarmed and too risky.
Homeowners who spot them are being advised to remove them immediately and report the incident to the Gardai.