Curious, very curious

My power went out the other night. Before it did, something in my house (I don't know what) let out 3 ear piercing beeps. It must have lasted 2 or 3 seconds. Here's my question...how did it know the power was about to go out in enough time to beep? I mean, I know - power surge, blah, blah, blah.  But still...it's like magic.