It's probably something to do with the water board, telephones or gas and the position of pipes etc.

There is a story about chaco marking for burglars, but it keeps cropping up when papers have nothing interesting to print, none mention paint markings.

Sometimes they reinterpret the articles with the same markings and make it about 'dognappings'. In one paper they asked various ex burglars and they said that there was no 'burglars association' and they didn't work like that.

Look at the pavement around or lampposts to see if there are other markings. That may give you your answer (and peace of mind) or see if there is any graffiti around in the same colour and it was just someone testing their can or cleaning the nozzle.