I saw something odd at ORD a few years back. I was at the gate waiting to board a flight to London. At the next gate was a flight to Delhi. Suddenly a herd of men in identical jackets with the flip down back panels revealing them to be CBP swarmed the area. Some carried bullhorns, others had passport scanners. They started shouting that all passengers leaving the US had to "check out." In English, only. The announcement was also made at the desk of the flight to Delhi. White, American types were ignored and the few that tried to show US passports to the agents were waved off. They only seemed interested in the Indians. As quick as they came in, all the men suddenly disappeared through a side door. The guys weren't there long enough to scan every Indian's passport.
ORD isn't my usual jumping off airport for international flights so I don't know how often this occurs. Last time I was there nothing strange happened.