If there is intelligent alien life out there it seems very probable to me that the asteroid belt is littered with sentinel probes that have been sent out to observe for particular signs of threatening or otherwise noteworthy activity in star systems.

2001 probably wasn't far off. I envision a small probe sent to each star system that finds a nice little hidey-hole inside an asteroid or moon and it burrows down in there and uses solar energy and local material to build more extensive infrastructure to replicate and send out copies to other bodies in the star system as well as to other local star systems.