Someone is responsible for it in every building. Nothing confirms they did it.
It depends on who is on duty — The shop supervisor one day, the guard the next, the last one out on a short shift. The routine changes, the switches don't.
Zone by zone — The hall, the storage bays, the workshops, the offices: each has its own switch, and one of them is always the one that gets forgotten.
Nobody is there to notice — A bay left burning at 23:00 is visible only to people who are not in the building.
The invoice can't help you — One meter for the whole site, so a zone lit every night never appears as a line you can act on.
A sensor on the lighting circuit reports one simple fact: is this zone under power right now.
On or off, by zone — Every circuit you cover, minute by minute, with the time each change happened.
Checked against your own hours — Evenings, nights, weekends, holidays: you set when a zone is supposed to be dark.
The alert goes to whoever can act — The guard on site tonight, and the manager who sets the rule for tomorrow.
Weekends are where it adds up — A zone left on from Friday evening to Monday morning is 60 hours of light for an empty building.
Know that the building went dark when it should have — and keep the record that proves it.