Constant low level alarm?

I've never used the alarm system, as it seemed too much hassle. However, I had a brief power cut yesterday, and the alarm went off, very loud. I flicked the switch down in the fuse box, turning the alarm off at the source, which seemed to fix it. But at 2am this morning, it started at a constant low level alarm which is driving me nuts. I can't see how to take the front panel off of the noise source, to remove the battery. Any suggestions?