I agree with the others who say it wasn't an IDB situation.

I've seen this happen several times, not just at ORD and not just with UA...but it does seem to be more frequent at hubs than line stations (regardless of the airline). I just keep an eye on the monitors. In the case of UX at ORD T2...they usually have the flights listed on the board behind the gate counter (I don't recall with the E Conc., but the F Conc. typically does for the gates which are being used for multiple flights at once). If I don't see my flight listed with the other 3 or 4 that are up there & departing around the same time, I ask one of the GAs.