Thank you all - appreciate your input. The doorstep is set well back from the road via a pathway so it's very unlikely to be related to street and utility repair. We're central East Dulwich and have interrupted burglars before (fairly recently), hence the heightened awareness of anything suspicious. I hope these markings were made by children playing, and I note the logic in what you're saying Rendelharris, and it does seem a strange thing to do to risk exposure but then the marking was pretty small and unobtrusive. The police say that they've not seen markings related to burglaries for a little while but they do have experience of this and there has been an increase in burglaries in the area. If anyone does see any symbols, please notify the police. Thanks.