Besides, if it wasn't a mouse, what else could it be? A rat's just a bigger mouse, right? Snakes wouldn't squeak. (Snakes in the walls is also the stuff of nightmares. Ugh...) Perhaps it's some sort of insect, but again, I don't know what bugs would squeak like that!
The good news is that I switched bedrooms, and the new room does not seem to be plagued by whatever makes the noise. I haven't heard anything at night, so I've been able to sleep peacefully again. But it still bothers me that one wing of the house is more or less off-limits because something is haunting it, and I can't figure out how to stop it because I can't even find it!