For a long time, we didn't have batteries in our smoke detector. (Yes, I know, I know. We're very, very bad. But we have batteries in there now, and have for a couple of months). When we did finally buy some batteries and stick them in there, we noticed a very annoying side effect. Whenever my husband would take a shower, he'd set off the smoke detector. To be more specific, it was not while he was in the shower, but afterwards.
You see, when I take a shower, I generally keep it pretty short and always leave the door open a bit, because foggy mirrors frustrate me when I'm trying to get ready. My husband, however, takes epically long, ridiculously hot showers with the door tightly closed because heaven forbid an ounce of heat escape. So, every time he opened the door to leave, a huge cloud of steam would exit with him and upset the smoke detector, causing it to go off.
This, on its own, was annoying enough. But Maddie, our darling dog, had lived her entire life in our house without the smoke detector ever going off and now it was going off randomly and without warning (according to her. Dogs don't make great detectives when it comes to cause and effect). She quickly came to the very logical conclusion that we must have a monster living in our hallway ceiling, and therefore she must bark and growl at the ceiling any time she, or one of us, entered the hallway.
I have since persuaded my husband to leave the bathroom door open a crack while showering, and the barking has abated a bit with the more infrequent upsets of the smoke detector. But she can't help but growl every time she passes under, just to let it know she's watching it.
On the plus side, I can rest easy knowing someone is always on the lookout for ceiling monsters.