Potentially related phenomenon: Across multiple locations >200km apart in France, I've personally noticed that mains powered lights reliably and visibly pulse a few times at the beginning of every hour, and half-hour throughout the day. From memory, it's roughly one or two 1-2s long sequences of pulsed dimming, at around 5-10 pulses/second. The pulse frequency doesn't seem to change, and it starts and stops abruptly. Does anyone know what this could be?
Generally this is a control signal injected into the power to control/inform loads about the current rate (e.g.: peak, off-peak), and other tasks. Pulsadis is a common system in France for this.
Perhaps, however Pulsadis uses a 175 Hz carrier, and neither that nor the bit periods are anywhere close to the pulse frequency I've observed. The amplitude of the dimming does however seem to be consistent with that of the Pulsadis signal.