I believe onstar only reads what you yourself could grab with a standard OBD reader.
There are other subsystems that have their own codes that a dealer can read.
These generally throw a code in the main system, though. e.g. P1E00 is powertrain requested MIL illumination (or similar wording)
But as said, it's probably a code that the computer recognized had corrected itself and turned off the light.
It may still be in history in the ECU, though.