The smoke detector...haunted

So, our smoke detector knows it is Halloween time. It started beeping yesterday morning, so I quickly grabbed a ladder and a battery. I changed the battery....and... it still beeped. All day...while doing my homework...BEEP....BEEP....BEEP! I had a headache by the time Craig finally came home. I figured he knew the special trick to get it to quiet. Well, he changed the battery (again)...and 20 seconds later...BEEP BEEP. We wanted to watch the Broncos game and worry about it later so Craig unplugged and removed the entire thing from the ceiling. We also removed the battery from it. This is when things got creepy. So there we were, sitting there enjoying the game...and....BEEP! BEEP! The smoke detector. It was removed the ceiling, sitting on our counter, no battery. Again..BEEP BEEP. 15 minutes later...BEEP BEEP. Where is it getting the power!?!? Short of throwing it out the window, we ran to the store, got a spankin new battery stuck it in, plugged it back in, problem solved. But...the question still remains..how does a smoke detector work with no power! It is haunted, I swear.