There is indeed a difference between "we have observed good evidence of X" and "there is something out there that, had we observed it, would be good evidence of X".
Even so, absence of observed evidence is evidence of absence.
How strong it is depends, of course, on how likely it is that there would be observed evidence if the thing were real. (I don't see anyone ignoring that fact here.)
For a moment lets assume there is some alien intelligent life on our galaxy which is older than us and that it have succeeded in creating super-intelligent self-modifying AI.
Then what set of values and/or goals it is plausible for it to have, given our current observations (I.e. that there is no evidence of it`s existence)?
Some examples:
It values non-interference with nature (some kind of hippie AI)
It values camouflage/stealth for it own defense/security purposes.
It just cares about exterminating their creators and nothing else.
Other thoughts?