Ding Dong

So this guy moved in next door to us last week - now we have neighbors on both sides. Cool! I haven't met him yet, but now twice, our doorbell has rung, and I answer the door, and NO ONE IS THERE.

But someone is standing at my new neighbor's door, presumably just having rung his doorbell. Are the wires crossed? Am I hearing things through the wall (no, it is super loud, in my house!) Is it possible that this is the fault of resonant frequency or some other physics-related phenomenon?

A mystery!