One sensor detecting a fire can be a false alarm. More than one sensor detecting a fire is very rarely a false alarm, and really ought to call the fire department.

Most automatic alarms are false. As far as I remember it's more than 90% in Denmark.

Accepting an error rate at 98% for Notre Dame should be a no-brainer.

There's a cost for deploying firetrucks, both in dollars and opportunity cost -- when you send 5 trucks to Notre Dame twice a week just in case the alarm is real this time, those trucks and firefighters aren't available to respond to a real fire

I dunno. Do you want fire trucks sirening through the city blocking traffic every day?

That's already a fact of life in every large city I've ever been in.

Do we know this to be true? If the alarm is going off for a good solid few minutes it might be worth a checkup.