What if no one knew what it was when it happened? During the flight, Passenger #2 and #9 start talking about the flash of light they saw stream by the plane. Once they land, they go to the authorities and tell their story. Said authorities start interviewing the other passengers and crew.
And since the pilot saw it he already radioed ahead and the entire crew and the passengers would have been sequestered immediately upon landing for questioning.
"What if no one knew what it was when it happened?"
It seems like quite a stretch to go from a flash of light to a SAM missle (I know you were just making a hypothetical). But I still think if it had any real credibility that airports would have been shut down.