A false negative occurs when an annotator fails to put a unit where she should, namely, where the reference (if any) tells us there should be a unit, and a false positive is the opposite situation.
no reference exists in the case of agreement measures, and there is a symmetry between false positives and false negatives: If annotator 1 puts a unit where annotator 2 doesn't, it is a false positive if we consider annotator 2 as the reference, or a false negative if we consider annotator 1 as the reference.