My cat meows at nothing

Cats see dead people.

I wonder if she's hearing something from the other side of the wall or if there might be some little critter inside the wall. Cats have amazingly acute hearing, and it's possible she's hearing something you can't because it's too high-frequency or you haven't noticed because it's so soft.

She could be meowing because she's frustrated: she knows there's something in the wall, but she also knows she can't get too it.

Demons

sounds like a cat to me

👻+ dont be scared because you/i don't know how ghosts look like,
i only seen it on movies & stories only
i cant belive in anything that i cant see
(maybe they would be good guys)

It’s a ghost

Cats have superb senses, even more heightened than dogs.

Cats can hear well into the ultrasonic range, or 100000 hertz, as opposed to our limit of around 20000.
Our feline friends also have superb smell, with 200 million receptors on their nose, as opposed to our 5 million.

Your cat is probably hearing some critter in the walls, or maybe even hearing, or smelling something that's outside your property.

To us, cats and dogs sometimes bark at nothing, but they definitely heard, or smelled something in the distance. Even in cities, there is lots of wildlife. Raccoons, squirrels, foxes, rabbits, stray cats and dogs, snakes, mice, rats, various birds, etc.

Mine do this when they are bored!