Some SETI scientists have conducted searches for radio signals or other
evidence of alien artifacts within our own solar system as well as looking
for radio signals from other star systems.
The idea is that, an unmanned alien probe may have entered our solar system
at some point in the past. This is made more likely if the probe is capable
of self replication. The idea of a self replicating machine was first
proposed by Von Neumann. A mathmetician and computing pioneer.
A self replicating space probe would spread through out the galaxy like a
virus. Making copies of itself from local materials once each star system it
reached.

It seems strange that SETI scientists are accepting of the idea of
Extraterrestial Probes or artifacts within our Solar system but not the
Earth's atmosphere.

I guess funding is so scarce for SETI programs and the credibility of SETI
is so often under question by the politicians responsible for providing that
funding that they go to great lengths to distance themselves from the UFO
debate.

Just read anything by Frank Drake, one of the founders of SETI for
contradictions in his belief in the likely hood of interstellar travel by
anything manned or not.