The other night, Rita started barking at about 2 am and woke us up. Since she quickly calmed down, I figured she'd seen a critter in the yard and we ignored it.
The next morning I found the problem: the smoke alarm in my office somehow fell out of it's little frame-thingy (sorry to get all technical on you there!) and hit the hardwood floor. How that didn't wake us up, I don't know, but it sure woke up Rita!
There was no battery in it, but I didn't really think anything of it since we'd had an issue last winter with the darn smoke alarms going off in my office/the hallway every time I took a really hot bath - which was, oh, about every night. So I knew we had dismantled a couple of them.
Later that day, I saw Rita freaking out about something by her toy basket. I thought, "Oh great, another unwelcome guest!" Turned out it was the missing alarm battery.