Under kW/h, the light will flash once for every watt-hour of power in use. Thus it will flash 1000 times per kW/h. Try switching off everything mains-powered in your house - the light won't flash. Switch on an electric kettle, and watch the light flash. If the light comes on at the start of a flash, and all house power is then switched off, it's possible that the flash doesn't complete its cycle, and remains steadily lit. That may be what you've seen in the other flat.