You can tell they're unscheduled when the box makes crackling noises and then suddenly bursts into speech without the customary lead in music.
Unscheduled messages tend to be important- fires, floods, typhoon warnings, missing old people etc etc.
"This is town hall. Some areas of Azusagawa are currently experiencing a blackout. Central Electric is working on the problem. We ask for your co-operation. Please be patient. I repeat..."
I can see how that would be a reassuring message if you were experiencing a blackout. Especially at night.
But, uhhhh, the in-house PA system?