What's with this?!?!

If it's not the kids getting us up, there's always got to be something else, it seems! The past couple of nights have been fairly restful as far as kids not getting up the entire night. Interestingly enough, however, our smoke detector has chosen the magic hours of 1:30-3:30 to begin "chirping." The first time it happened, I woke Bryn up & he tried to take the detector down. That's when we discovered that it is hardwired into the house. Interesting. He had a heck of a time getting the thing to snap back into place...it's a wonder no kids woke up! The chirping then stopped. But, same thing happened last night, this time around 1:30. Grrrrr. It does not make this sound during the day. This is the oddest (and most annoying) thing!!!