On Fri, 29 Jun 2018 17:14:01 -0700 (PDT), RichA
wrote:

So they send a mission to Enceladus. They find no life. "Ah well, maybe the equipment isn't good enough yet?" 10 years pass, another mission is sent. This time they find bacteria and viruses. Now, was this missed the first time around, or was it evolved contamination from the first ship?

https://www.bbc.com/news/science-environment-44630121

All life on Earth evolved from a single common ancestor, and shares a
huge amount of common genetic coding. Anything we leave behind will be
readily identifiable as originating on Earth. Even if alien life uses
the same genetic chemistry as Earth life, there's no chance it's going
to code for the same genes (and especially for all the inactive
segments).