So let me explain what I think it means. It is a complete absence of evidence, not an absence of some evidence we would expect to find, if the proposed entity was present.
Until relatively recently we lacked the technology to detect exoplanets. That was an absence of evidence, not evidence that exoplanets did not exist. Similarly the Oort Cloud is not - yet - observed, but that is not evidence that it does not exist. (Although Young Earth Creationists have claimed otherwise). Again, the reason is that observation is too difficult.
Or another from this forum. There is no official Roman record of the execution of Jesus. Since we do not have Roman execution records from Judaea covering that time, this is an absence of evidence that is not evidence of absence. The records - the evidence - are absent. If we had a complete set of those records - but the record for Jesus was missing - it would not be an absence of evidence. In that case - where we would have a strong expectation of finding the record - it would be evidence of absence.