I'd like to have a post disembarkation de-brief with the SLF and ask them what they were waiting for. . . . the landing ? out of curiosity to see how it panned out. ?
Of course the time scale could have been very short, between take-off & the crew being notified by ATC that they had left something behind, but I would have thought most pax would have seen fit to alert the CC P.D.Q, after one of these round black things was seen flying off.