I always keep my passport on me. One of my friends once witnessed a failed theft attempt aboard a trans-pac flight (thief tried to steal a laptop out of an overhead bin bag, but dropped it on the pax who was the owner...)
There's also FT stories of laptops disappearing, etc. These days.
That said, I'd guess that these passports were scavenged and collected/compiled. Is it that implausible? Not everyone is as "ick" about dead bodies...
Exactly why I keep my stuff on me or in my view at all times... You can't trust anyone nowadays.
I'm sure it's completely plausible that they're scavenged or removed from personal belongings, but why? Nevermind...there really doesn't need to be a why.