Agreed. No better explanation... so far.
Indeed, I was worried the computer might rebook us on a later flight... as a "courtesy."
But if a passenger reported a lost BP, the GA would know the passenger's name and seat. My BP had my name and my seat, and my BP was not reported lost. Likewise neither was my wife's. Moreover, like I said, the GA asked no one else in line, not in my earshot (three or four passengers in front of me). But if I/we were boarding pass imposters, I could have put the passenger's name to memory. In which case the gumshoe GA would have failed to catch me. However, if he had asked an imposter for info
not on the BP (such as DOB) that would have worked. But do GA's have access to such info? In any case, I don't think lost-boarding-pass is a good explanation.
Come to think of it, he did ask me "What's your quest?"
If there's concern about a boarding pass that was lost in the airport, isn't a better solution to flag the person's reservation so that they're required to show photo ID to board? It's pretty normal for agents to ask for ID, for instance when you check in, check a bag, or have your boarding pass reprinted.