I finally figured out what it is. We have two of them (which I hadn't noticed before). One in the dining room, which is really just a big room that connects to the living room and porch, and one in the kitchen. We have a front and back door...two doors. Two mysterious sockets in the wall... get it yet?
On a side note, we have no plugs in our dining room. It's a pretty big room, and the only room in our house with a table. So if I wanted to, say, plug in my sewing machine at the table, I would have to run an extension cord. The walls are concrete, so it's not quite as easy and just running some electricity over (okay, I'm not totally sure that's easy in any other type of house, but it seems harder when the walls are solid concrete). I thought this mystery plug thing had to be some sort of socket at first. It's not.
So what is it? It's our doorbell!