Lights turn on or off by themselves
Lights change with nobody near a switch. It feels like the system has a mind of its own, and the first thing to establish is whether the behaviour is actually random or merely unexplained. Those have very different causes.
Start by keeping a note for two or three days: which lights, what time, what the weather was doing. That log is worth more than any guess, because the single most useful split is regular versus irregular. Something happening at 6:42pm every evening is a schedule doing exactly what it was told years ago. Something happening at unpredictable moments is hardware.
Nothing on this page asks you to take anything apart. Every check here is done by looking, or by pressing buttons that are already on the outside of a wall plate. Opening the hinged door of your switchboard to see the breakers is normal and fine. Removing the panel behind them is not, and neither is unscrewing a wall plate or lifting the cover off an enclosure.
In Australia it is illegal to work on fixed wiring without an electrical licence, and the reason is not paperwork: the terminals behind a C-Bus wall plate can be sitting at mains voltage. Anything past the faceplate is a job for a licensed electrician, ideally one with C-Bus accreditation.
Is there a pattern to it?
- It happens at consistent timesSame time each day, or shifted by an hour since daylight saving. This is a schedule or an astronomical timer still running, quite possibly one nobody has ever seen.
- It is genuinely unpredictable, and one plate is involvedA failing input unit can send presses that nobody made. Outdoor and bathroom keypads that have taken moisture are the classic offenders.
- Lights flicker, buzz, or drop out rather than switching cleanlyThat is not a phantom command. It points at the dimmer channel or the lamps rather than at anything sending messages.
- It started when a controller or touchscreen was replaced or removedLogic left running in a controller, or logic half-removed, can drive behaviour that no switch explains.
The two everyone forgets
First, motion sensors. C-Bus installs from the 2000s often included occupancy sensors in hallways, garages and bathrooms, and after twenty years of repainting they are easy to stop noticing. A sensor with a widened field of view, or one that now sees a heater or a moving curtain, produces exactly this symptom. Second, an astronomical timer: a schedule tied to sunset rather than a clock time drifts through the year by design, which reads as random until you plot it against sunset.
Water is the underrated cause
Moisture in an outdoor or bathroom keypad is the most common source of genuinely random switching, and it tends to appear after heavy rain and settle in dry weather. That correlation is the giveaway, and it is another reason the log is worth keeping. If the behaviour tracks the weather, note it and say so when you call: it points straight at a specific plate.