Since this is recirculating in the public consciousness, I'm going to repeat my pet theory: they're automated Von Neumann probes that have been in the solar system (and probably ~all star systems in the galaxy) for longer than humanity has existed, and are performing reconnaissance now that there are signs of intelligent life on the planet.

I like this theory because it obviates large problems with other theories, ie crossing interstellar distances just to visit us (they were already here), the lack of observed Von Neumann probes when they should be common (now they've been observed), the idea that some earthly nation has managed to develop technology so far beyond all others and keep it secret (nope), etc.