(Somewhat of a repost, since I told a very similar version of this story in a comment on "Night At The Stanley")

When I went to high school, we used to have choir concerts, accompanied by piano, in a hall that used to be the mansion of a wealthy man. Its acoustics were perfect; you could hear the slightest footstep.

One night, I was working late in the hall in an office on the second floor. It was after eleven, and I was tired and getting ready to call it a night. It was weird being there without the noise of other students.

Without warning, the piano downstairs sprang to life. Random notes rang out as though someone were smashing down on the keys. It was over in a second, and the hall fell silent again.

I was frozen solid. There was simply no way that anyone could be down there. I hadn't heard a single footstep– in fact, I hadn't heard a sound for hours. Worst of all, I couldn't run– the only way out of the building was right by that piano. So I gritted my teeth and made myself walk out and look down onto the first floor, telling myself that the first levitating object I saw would send me running…

As I reached the railing, I saw a black cat dart out of the shadows and silently pad out the open door into the moonlight– a cat who had almost certainly tried her paws at piano playing a few minutes earlier. If I hadn't made myself go and investigate, or had waited just a little longer, then I'd never have seen the cat.

No, I can't prove that it was the cat that played the piano, but it does seem a very likely explanation.