Big difference.
You don't know how much money is in my wallet. I do. You have no evidence, and you don't have a means to detect it, but it doesn't mean there is no evidence to be had.
That third little star off the end of the milky way may be a gigantic alien beacon transmitting a spread spectrum welcome message, but we just haven't identified it as such, or spent time trying to reconstruct the message from the spread spectrum signal.
We see it. We record it at observatories every night. But we haven't identified it as a signal, nor decoded it.
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?