> 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_Lights http://en.wikipedia.org/wiki/Belgian_UFO_wave