last night, our neighbor's fire alarm went off after midnight. we live in a town house and the neighbor's house is empty while it's being renovated. i couldn't reach the owner, so i called 911.
turns out, there was no fire. the electricity had been cut and the fire alarms, which are electronically wired, were operating off their back-up battery. the battery was low, so the fire alarm sounded as a warning.
lesson: even if you have electrically wired fire alarms, you still need to check and change your batteries regularly. God forbid there was actually a fire next door in the middle of the night (which is exactly what i thought when i was awakened by that shrill sound).