Humans are still unable to comprehend the limits of their abilities to perceive what is around them. Granted, their expectations of extraterrestrial life may be high and we are more likely to discover single-cell or simple organisms than beings of higher intelligence. But given the infinite number of stars with their planetary systems, would it not be more probable that there are civilisations akin to ours or even more further technologically advanced? Or that these more advanced beings have already discovered us but choose not to reveal themselves outright?
What if our probes have actually encountered extraterrestrial lifeform but they cannot detect it because they do not have the same life-signs as we (Terrestrial lifeform) do?
Like, they do not emit heat, do not breath, and move differently compared to us?