Learn Hardcore Greek: Άναψε ο ανιχνευτής καπνού, με αποτέλεσμα να ελέγξει ο υπεύθυνος ορόφου. - The smoke detector lit up, resulting in the floor warden checking.

Its basic literal meaning is lit up or turned on, but with something like ο ανιχνευτής καπνού it is understood as activated or went off.

So here it does not mean someone physically lit the detector. It means the detector became active.

Why does the sentence start with the verb Άναψε instead of the subject?

Greek word order is much more flexible than English word order.

So both of these are possible:

Άναψε ο ανιχνευτής καπνού
Ο ανιχνευτής καπνού άναψε

Starting with the verb is very common in narration and often gives a more event-focused feel, like The smoke detector went off...

The subject is still clear because ο ανιχνευτής is in the nominative form and has the article ο.

Why is it ανιχνευτής καπνού and not some other structure?