I disagree with some previous interpretations. "Will" in the first sentence denotes epistemic modality, that is the degree of commitment of the speaker towards what s/he is asserting ("you have probably seen that..."). Now, this use of "will" is quite restrained and rare in English vs. other auxiliaries such as "must" or "might": "I see his car. He might/must/will?? be back". That is, "will" is not thoroughly grammaticalised as an epistemic auxiliary in Eglish differently from other languages such as German (see above).
The second example, instead, relates to a future perfect which presents an event as having already taken place in the future without any certainty as to whether it will take place. This is obviously a huge, but not transparent conceptual clash since we don't analyse constructions, but acquire and replicate them.
The only feature the two examples share is that of "irrealis", i.e. of something that cannot be factually assessed.