If They Are Here, Where Are They? - Observational and Search Considerations
Robert A. Freitas, Jr
This paper discusses observational considerations in a search for extraterrestrial intelligence (SETI) program to detect extraterrestrial messenger probes in the solar system.

Interstellar Espionage: While We're Watching Mars, Could Someone be Watching Us? (Interstellar Probes)
Seth Shostak, Special to Space.com
Have aliens sent mechanical emissaries to our solar system -- robotic probes on a snoopy mission to reconnoiter Earth? It’s certainly an intriguing idea: sophisticated spy satellites from light-years away monitoring our planet, watching the slow evolution of life, and reporting back to their alien masters. Such a scenario has frequently appeared in the SETI literature, and Allen Tough, at the University of Toronto, has urged that we take the idea seriously enough to make a search for these alien "bugs." R

Interstellar Probes: A New Approach to SETI
Robert A. Freitas Jr., Journal of the British Interplanetary Society, Vol. 33, pp. 95-100, 1980
Interstellar transmissions via energy-markers (photons) or matter-markers (probes) appear to be energetically indistinguishable alternatives for advanced technical societies. Since only Type II and Type III civilizations realistically can afford beacons or starprobe technology, alternative distinguishability criteria suggest the possible superiority of intelligent artifacts for contact and communication missions among extraterrestrial cultures. A balanced, more cost-effective Search for Extraterrestrial Intelligence (SETI) strategy is proposed.

Resources on Interstellar Probes
Interstellar-probes.org
This page contains a guide to the resources needed to study interstellar probes, interstellar travel, interstellar propulsion systems, the search for robotic interstellar probes or ETI probe visitation, and information on earth's robotic probes. Abstracts for the papers or the introductory text from the book, or book chapter, will be given. R