Usually when there is a gate change for the smaller planes, they're able to narrow down rather quickly who has not yet boarded.

It's just common sense to see that 49 of 50 people have boarded, the remaining person is checked in, and there WAS a gate change. Gee, maybe make a quick announcement at the original gate?

Of course, UA can flip the coin on you...saying "gates are subject to change."

Either way, the UX operations at ORD are so hectic and disorganized that one really needs to be on their toes while traveling in that territory.