Von Neumann probes could reach every star in the galaxy pretty quickly. A civilization with a few million years start on us should have their robotic friends here already, basking in our sun.
And there only has to be one civilization that 1) can and 2) does create such probes. If spacefaring civilization is common, it seems awfully unlikely that none of them have done so yet.
> it seems awfully unlikely that none of them have done so yet.
Based on what? How did you estimate the probability of a random intelligent civilization sending detectable (from Earth) Von Nuemann probes into the universe? For a probability of 1/1000 there could be hundreds of intelligent civilizations within our galaxy without any such probes, and the probability can be much lower than that.
> it seems awfully unlikely that none of them have done so yet.
We don't know that, there is no way to detect that with our current technology unless one of the probes land on earth and the government/military agrees to disclose and confirm the information, maybe this already happened but nobody wants to see it because they are afraid of the implications http://en.wikipedia.org/wiki/Phoenix_Lightshttp://en.wikipedia.org/wiki/Belgian_UFO_wave
And there only has to be one civilization that 1) can and 2) does create such probes. If spacefaring civilization is common, it seems awfully unlikely that none of them have done so yet.