Police are investigating reports of symbols scratched on house intercoms that raised the alarm for Sliema and St Julian’s residents over the weekend.

A photograph of an intercom in St Julian’s with a doodle that looks like a number six was posted on Facebook on Sunday, triggering a discussion about similar symbols used by criminals abroad to help other burglars know which homes to target.

When contacted, the person who posted the photo said he knew of similar cases in Sliema . He believes the people who made the mark drove off in a green van bearing the number plate CO2478AX, which the police told him was fake. He believes the registration is Bulgarian.

He said he noticed the van in the afternoon while studying. He waited indoors, keeping an eye on the left-hand drive, small family van that remained parked in front of his garage for about 30 minutes.

The sound of him opening the blinds alerted the driver, who was standing outside the van. The person got in and drove off with two passengers.

The family is now making sure there is someone at home at all times while raising awareness on social media and considering starting a neighbourhood watch. According to media reports abroad, the symbols in question indicate whether a house has already been burgled or is worth targeting. They could also indicate whether a woman lived alone, if the residents were easily alarmed or whether there was a dog.

It is similar to a system developed in the US during the 19th century by migratory workers, known as the hobo code. To cope with lifestyle uncertainties, vagabonds would leave a coded symbol in chalk or coal to give directions and warnings to others in the brotherhood.

The discussion about such secret code in Malta intensified after sections of the media received some more pictures of marked intercoms.

Acting Police Commissioner Ray Zammit said the police were looking into the reports.

Patrols in Sliema and St Julian’s were being strengthened and he urged people with any information to contact them.