In my previous flat, there was a smoke alarm and a Carbon Monoxide alarm. Got home from work one day to find the CO one beeping. Took it off the wall, the instructions on the base said if the alarm sounds, call the Fire Brigade. As I clearly wasn't dying of CO poisoning, I didn't want to call 999, so looked up the landline number of the local fire station. Spoke to a nice chap, explained the problem and that it wasn't any kind of emergency. He said he'd send someone to take a look. I imagined one person in a car, then 5 mins later I heard the sirens coming along the main road ...
Two fire engines, firefighters all togged up. About three of them came up to my flat, the rest hung around outside. I kept telling then it wasn't an emergency and I hadn't called 999! Mortified! It must have been a quiet shift and they were bored.
P.S. The CO alarm was at the end of it's usable period, they said to chuck it away.