Question:
About a year ago I bought a battery-operated smoke alarm (model EI 100BN) but never got around to installing it. With a view to doing so, a week ago I added what I think was a fresh 9V PP3 battery and left the unit on a shelf in the garage. Since then I've noticed that apparently at random it gives off a single short beep - loud enough to wake light sleepers like my wife and me I reckon. It's not the 3 or 4 beeps you get when pressing the Test button or immediately after installing the battery. I thought it might be a low battery warning (albeit hardly a very effective signal), but that reads 8.98 V which presumably should be OK.

It's an 'ionisation type', and the sensor is marked as radioactive (Americium 241 0.9uCi), so I'm even wondering if it's detecting some occasional radioactive emissions! I'm going to rig it up to an isolated sound detector to gather more data (such as whether it beeps only when I'm around...), but meanwhile would appreciate any suggestions on the likely cause please.

Answer: The battery is weak. Smoke detectors emit a short beep every 30 sec - 3 min. to signal when the battery should be changed. I know this because the 6 or 8 units in my house always start to beep at about 3am, when the house is coldest and the battery voltage is lowest. The problem is locating which unit is beeping. Since the burst is so short, and the pitch so high, and 6 or 8 units in the house, it is very hard to tell exactly which one is beeping, especially at 3 am when I'm not completely awake.