My smoke alarm was very sensitive and would go off all the time. A few weeks ago after it went off it started making a single bleep every 30 seconds and it's still doing it! It's mains connected and even when i turn it off it still bleeps. Also it hasn't went off since that day. I've tried to get the cover off but can't. What's going on?
Most of the mains driven units have a 9v battery back-up in case the mains goes off. Maybe that's the signal that the battery needs replacing? Different models / manufacturers vary.
Some alarms have a cover that slides off rathers than unscrews, have you tried that?
Nothing wrong with the unit - You just need to get the cover off - But I bet if you look carefully, there will be a wee drawer to pull out with the battery to replace!