What is Evidence Of Absence?

Legal Definition
Evidence of absence is evidence of any kind that suggests something is missing or that it does not exist.

Per the traditional aphorism, "absence of evidence is not evidence of absence", positive evidence of this kind is distinct from a lack of evidence or ignorance of that which should have been found already, had it existed. In this regard Irving Copi writes:

In some circumstances it can be safely assumed that if a certain event had occurred, evidence of it could be discovered by qualified investigators. In such circumstances it is perfectly reasonable to take the absence of proof of its occurrence as positive proof of its non-occurrence.
-- Wikipedia