The doorbell is across the hall. Old doorbell? The other side of the wall has a solid outlet cover.
If the cover is solid, it is not an outlet, it is a cover!
As Glennsparky said, it is the transformer for your doorbell. It doesnt matter where your door bell is, I've seen them in closets, attics, crawl spaces and just about anywhere they can hide them! Next time your in a home improvement store, visit the Electrical Dept and ask to see a door bell transformer for comparison, or
Take our word for it!
Still not convinced? Unhook one of the wires in the picture and see if your doorbell works? (providing your doorbell is currently working)