It is arguably the case that, due to the immense distances between stars, any evidence we discover of extraterrestrial intelligence, whether it be an artifact or an electromagnetic signal, may come from a long-vanished civilization. Thus the entire SETI project can be seen as a form of archaeology. Additionally, due to the extreme age of the universe, there may be a reasonable expectation for astrobiology research to produce evidence of extinct alien life prior to the discovery of alien life itself.
https://en.wikipedia.org/wiki/Xenoarchaeology