I think I've seen this at Terminal 4 at JFK
flights, but a similar system is used at other international airports I've been to. On the departure monitors located in the concourse, each flight has a "Status" column in addition, to airline, flight no, destination, gate, etc. In Status, the options are On time, Delayed, Cancelled, Security/Immigrations, Boarding, and Departed. It has to become part of airport culture to not let passengers go to the security lines unless the status of the flight is security or boarding. Now that I recall, I think I saw this at BOM the last time I flew out of there. Of course, the BA
flight my sister was taking was going stated as going through immigrations and security while the flight was pulling-up to the gate next to ours. Once you pass security at BOM, you're in the boarding lounge and there is NO going back outside to do some duty free or get a drink.
Of course, many airports like PIT
rely on passengers clearing security and spending time in the concourse or at the "malls" that seem to be popping-up everywhere. Frankly, I'd rather pass security early, drink a latte, and go find a place at the end of the terminal where I can watch arrivals and departures.
You land a million planes safely, then you have one little mid-air and you never hear the end of it - Pushing Tin