No, not impossible. One shows as a through passenger once one boards the inbound flight. There are still quite a few International flights into ORD--some of them quite lengthy. As long as they didn't have a 24+ hour open jaw in ORD, they would show as a through passenger as soon as they boarded the flight in Timbuktu or wherever. The time zone thing does strange things sometimes as far as the 24 hour advance check-in--especially if you cross the International Date Line in the process.
I'm not saying that there isn't some monkey business going on, there could be. But, if what you said happened actually happened, you need to file a complaint with the appropriate travel department. Though the system is programmed very well and is adept at catching no-nos, it's not perfect. A glitch in the system or an unexpected situation may need to be brought to the attention of those who manage the NRTP.
Addition:
Chris I just read your post about the D1T originating in DTW. Aside from the fact that the agent could lose his/her job for disclosing to you information from another passenger's PNR--you really have no right to know anything there--this sounds like a situation that needs a formal complaint. I don't see how that could have happened otherwise.