PaxBoy
The problem is someone could just print a boarding card at home and not go to the airport, it would still showed checked in, just like someone who went to the terminal. There is no mandatory reprinting of online boarding cards anywhere that I've ever seen.
There isn't any way for them to know if you were at the airport or not, unless the airline made some kind of note or offloaded anyone who didn't physically make themselves known to an agent when a flight was disrupted. As far as I'm aware this doesn't happen as standard. The only time it would is if 1 flight was cancelled and the passengers were manually rebooked to another service. The PNR would show the changes and thus anyone who didn't show up to be rebooked would not have been at the airport, so wouldn't be able to claim.
When the airline sorts out claims they could clearly see who is entitled to what. Even if someone booked a flight on another carrier or another mode of transport and didn't contact airline staff, they should be entitled as they clearly intended to travel, but they took it upon themselves to re-route and could provide that information to the airline in order to claim,.
Someone who did neither and had no proof that they re-routed themselves could be looked at with suspicion as to whether they even wanted to travel in the end but then I do wonder how such a case would stand up in court.
MidlandDeltic
This kind of thing is being rolled out at some airports hereIt is probably in place at a few places world wide already. Certainly LHR T2 will make use of it and I think T5 does. It is used so boarding agents can see who actually passed through security. At -15 (or whatever the cut off is) anyone who is showing as not cleared security can be offloaded without delaying the flight. Anyone who is through and not yet boarded would show as such, and the usual announcements and checking seats could take place. Once a flight is gone the data is all deleted though. All It shows is, from what I've been told, Sequence Numbers, Names, Time they passed security, where they passed and what the last information the passenger had was. By that I mean, on the little screen above the boarding card scanner at the checkpoint, it would show something like "Check screen for gate" or "Go to Gate 123" for example.