Interesting. Do any online booking agents accept these codes? I tried with a few, but they didn't recognize them.
To the OP - CHI, BER (berlin), BUE (Buenos Aires), BUH (Bucharest) and LON are some other multi-airport code. You could go to the
Wikipedia article for IATA codes and search the lists for the term "All Airports."
Also, according to wikipedia- QDF (Dallas-FW) and QHO (houston) are some other codes in the US.