Absence of proof is not proof of absence, and Sagan should have said that.

Absence of evidence is evidence of absence if evidence was sought and not found, and much of science is based on this. Or if evidence of presence should be expected ... consider for example the absence of evidence of an elephant in your living room.

This saying should die along with "you can't prove a negative"--Euclid proved that there is no greatest prime over 2000 years ago. What can't be proven is a universal empirical--positive or negative--such as "no raven is white" or "all ravens are black".