I agree that's a little odd. Normally, trouble codes remain for longer than that, even after the lamp goes out, unless the battery went dead or was disconnected.
Most faults require two successive failures to turn the lamp on, and then three successive test pass results for the lamp to be turned out.
Certain important faults will set the lamp at the first fault.
If there's only been one fault, the code is stored as "pending" and will be erased if the next test passes. I suppose it's technically possible that this was an important fault that only occurred once.
Of course, it's also possible that the technician didn't check all necessary systems (engine and trans) or for whatever reason decided the thing to do was to clear whatever the code was and simply deny its existence since the check engine lamp was on.