The fact is, not finding the contact lens in front of the bar IS evidence for its not being in front of the bar. Absence of evidence is evidence of absence.
NO. Not finding the contact lens in front of the bar is
not absence of evidence, it's evidence of absence. Reducing to the rather tautological evidence of absence is evidence of absence.
Trying to relate this to the "alien thingy", let's not forget that SETI has been "searching space" for decades without success
I think you're rather over-estimating the efficency of the SETI search, it can scan only a tiny, tiny part of the sky at one time, and it can only pick up a tiny portion of potential signals. It's search is more like picking a hundred random grass blades in the football example above.