Alarming logic

I am faced with a small but satisfying logical puzzle in my office at Bristol University. When I come in first thing (actually, even if I come in about 10), the alarm is often set. In fact, the first time I ever entered the building the blasted thing started beeping at me, and no one had bothered to tell me there was an alarm. So now, as I belatedly know the code, I unset it. But the puzzle is - how and when does it get set?

I certainly never set it on leaving. I wouldn't know how to, and anyway I have no way of knowing if the building is empty. It's a tall, old house - my office is on the second floor and I can often spend the entire day here without seeing another inhabitant, though I regularly hear them. The same uncertainty must surely apply to any ordinary resident. So how is it done?

In principle it could be automated. To be safe, there would have to be motion sensors in every room, which as far as I can tell there aren't. So if it is automatic, perhaps they just assume there's no one here after, say, seven pm and set it. Pity the person trying to put some serious work in. The alternative is a security person pops in and does it. But if so, would he or she really bother to ascend the four flights of stairs to my room, night after night, when there's never anyone here? I somehow doubt it.

So it looks like it's likely to be a case of set the alarm and fingers crossed. Remind me not to work late.