There have been a couple of reports of incidents recently where either the door bell has been rung, or the knocker knocked, and when the householder has gone to the door, no-one is around. Incidents have been early evening, but there was one report of this happening at 4am.
If you are not expecting someone, please either look through the peephole in your door [if one is fitted] before you open the door, or have a chain in place so that the door can only be opened to a limited degree.
It is thought that burglars are possibly using this ruse to see if a homeowner is in residence. Another ruse is to switch off the electricity at the mains box. As mentioned previously, you can obtain a lock for your mains cupboard from the Electricity Board who have a universal key so that they can access the cupboard for meter readings.