Rhaom

I'm going crazy here. I just moved into my house 2 months ago and the first thing I did when i moved in was replaced the batteries in all the smoke detectors so I know they are good. However, early afternoon today suddenly they started beeping, roughly once every 2 minutes and its been going on for about 9 hours now. I've tried taking the batteries out one at a time, all 3 at the same time. I've tried turning the mains electric supply to the smoke alarms off and i've even had all 3 alarms detached from the mounting plate, with the batteries out and the mains electric turned off and they STILL continue to beep and its driving me completely insane! If anybody has any insight as to what I can do to fix this issue I would greatly appreciate the help