Interfaith Group

100% certainty that there is no unicorn in the room. There are always scenarios that would account for its being present, but your not detecting it (it was invisible, its odor was being masked, someone used a shrinking ray to make it too small to see, etc.). In such a limited environment, the scenarios have to be so outlandish as to defy credibility (though no more outlandish than those used to explain away supernatural/religious hokum all the time), but technically you can never be "certain", even if you can be for any practical purposes.

And yes, the broader the stage on which you want to assert "absence" the less certain you can be, though that is still different than asserting a lack of sufficient evidence to conclude "presence". But the stage for claimed evidence for "god" is much more limited than that for alien intelligence. Many religious believers assert the absence of most (or nearly all) gods, and are entirely justified in doing so.