Mirage
what did the Mirage pilot see?
He would have come close enough to look into the cockpit. He would have seen enough to know there was no explosive decompression. Perhaps he saw a single pilot and which seat he was in, perhaps looking back at him, it would after all be hard not to stare at a fighter jet moving in close.
whether he saw and what he saw we don't know yet but the authorities do. Reports are filed, film watched. Perhaps this is corroborating what they are hearing on the CVR and giving additional ability to conclude what they are concluding. Perhaps....