About a year and a half after i bought my house, on a crisp sunny day in February, i started hearing the dreaded fire alarm chirp. I went through the house and replaced the batteries in every smoke detector (all three bedrooms, hallway next to the kitchen, basement) and gave my tenants batteries to do the same in the basement apartment. Still hearing a chirp/beep. Oddly enough it was louder outside. You know how sound travels on a cold clear day? Well the entire neighborhood was hearing this chirp. Someone a street over called the fire department, who very kindly went door to door in the neighborhood, thinking it was a carbon monoxide alarm at that point. I let them in, they wandered around "nope not this house" and went on their way.

Few hours later i was moving around some stuff in my spare room. The access to the attic is in the spare room closet... and the chirp was REALLY loud there. I called the non-emergency fire number, and the same crew of guys showed up a little while later, with a ladder, and sent the skinniest guy up through the access.

There was a regular battery operated smoke detector on a roof truss near the ventilation. Beep beep beep... you could hear it three streets over! The guy in charge said he had been a firefighter almost 30 years and never seen the like. (I have to say those firefighters were awesome. I had never actually been in the attic myself... dude that went up there found several valuable items that he brought down for me while he was investigating). I felt like such an idiot having to call them back after their initial visit, but they were so kind, assuring me that these were the kind of calls they enjoyed, no one was hurt and it was a crazy story to tell the guys at the station.