I think it's fair to say that when people ask what are the odds of something happening, they're talking about the hypothetical scenario where something similar happens a bunch of times, and asking out of those trials, what proportion will have this certain thing happen.
The trouble you quickly get into is that you have to specify what exactly you're asking:
* What is the prob that this particular guy's passport would be found intact, at that exact location, by some guy who hands it to a police officer? Pretty low.
* What is the prob that any photo ID of any of the hijackers would be found before the buildings collapsed? Pretty low that, too, but certainly greater than the first scenario.
* What is the prob that any hijacker belonging is found? We're starting to get a little higher.
As you get less and less specific, the probability gets higher and higher, and you realize that out of the very large set of astounding coincidences that could possibly happen, it's not too remarkable that one of them did in fact happen.