Beep (pause 5 seconds) Beep (pause 5 seconds)

I woke up at 4 AM yesterday morning to this sound. Actually, the cold was blowing in so I went through the house to close windows. Beep.....Beep.....Beep When I realized that no one else was there to check it out, I got up and went to the hallway. I stood under the smoke detector and continued to hear it. I go out into the laundry room to get the ladder (good thing I keep it indoors), climb up and remove the battery. Go to the kitchen and dig for a 9V battery... I finally found one. Climb BACK up the ladder, install the battery, close the ladder, put it back in the laundry room, start walking back to bed and ... BEEP.....BEEP.....BEEP ~~ WTF? so I stand there like it's going to say, hey tobey, it's me beeping. Remember it's 4 AM. I somehow remember that I have a carbon monoxide detector in the hallway but it's plugged in. I unplug it - the beeping stops. Why does it have a battery AND plug into the wall?