Here’s a little bit over at The Register related to the Fermi Paradox:

The problem, according to boffins Jacob Haqq-Misra and Ravi Kumar Kopparapu of Penn State uni, is that it’s entirely possible that our Solar System is littered with ancient alien space probes and we simply haven’t found them yet. Haqq-Misra and Kopparapu have investigated this mathematically.

Of course it might also be the case that we have no idea what they might look like. Maybe they wrap them up in a big black rock the size of an aircraft carrier…