It's usually late at night or maybe you hear it in that short silence between songs on the cd. It's a scratching noise or a sound like tearing paper. You can't tell where it's coming from right away because it seems to have an illogical locus high in the air in the middle of the room. Maybe you turned the music or the tv off a few times because you swore you heard it again, but you can't catch it when you're listening for it. Then when you're lying in bed, maybe holding a paperback propped on your extra pillow or just dozed off enough to get a little drool on your cheek, you hear the unmistakable sound of aliens ripping through the fabric of spacetime specifically seeking your lame corpus to bring you back to their world. Then you realize it sounds more like a cat.
The cat sounds like it's right there in the corner, but there's nothing there except dirty clothes. You even check under the dirty clothes, but without putting them in the hamper, and come to the conclusion there must be a ghost cat in the house. Your ghost cat yeowls for another half hour or so and fades back into the nether regions where animal's souls are caught between dimensions that only John Edwards can access.
The next day you ask your roommate if she has heard the ghost cat and she says no and that you should stop bringing your beer with you into the shower. That night is uneventful, no aliens, no cats, not even extra stink from the clothes so you wonder if you imagined it all. But the following morning brings vindication because your roommate has freaked-out eyes as she tells you that she was kept up all night by a phantom meowing.
You discuss the possibility of staging a catty exorcism and what things you might need for that: squirt bottles, a trinity of dogs, some blessed catnip. It seems like a pretty straightforward affair until you hear a crash somewhere in the vicinity of the stove and then a frantic scrambling...up the wall and back down again, scratch, scratch, scratch up...and nails sliding back down. I'm sure I've seen this happen in cartoons involving cats and greased telephone poles, although I've never in my life encountered an actual greased telephone pole.
So you call your landlord and tell him you've got a cat stuck behind your wall. He tells you that can't be a cat you heard. Must be a squirrel. You tell him you heard it meowing. He doesn't care, the access is too small for a cat, must be a squirrel. You tell him you're aware that some animals can mimick their predators, but there's no way a squirrel can sound like a cat unless it's a squirrel playing a tape recording of a cat. In which case, he probably also has a gun so you'd both better just leave him alone.
At some point, maybe you, maybe the landlord, will suggest that someone will have to knock a hole in the wall to get it out. This is not necessarily true. Most of the time, your local Animal Protection Society or your veterinarian will have some handy traps that you can use to safely extract the errant squatter. Most places don't charge you to use them. The trap is basically a cage that you bait with some food. The animal goes in to get the food and the door closes behind him. Remember to check the cage often! That peanut butter cracker you baited the trap with has long-since been eaten. You take him to the woods, the vet, the APS, mom's house, wherever seems most appropriate and let him out. If he's pissed off you let him out fast and run away before he can get to his gun.