I'm baffled by these "happened to be standing by..... and noticed" tales.

I travel a lot, I see manifests, I'm not incurious, and I'm not sight-deficient: yet I've never gleaned any information from those documents. I suppose i'm hampered, a little back-footed, in recognising that the list, while not classified, isn't there for my inspection,

So I wondered when these snoops takes place (on boarding, galley visit, lavatory trip, dedicated recce) and how much time "just standing by" the document is needed, and how much attention, scrutiny, is required to come up with the sort of information the OP acquired.