Fault codes are NOT stored . . .
after they are erased! Once they are erased, they are gone!
Regarding CE going off by itself: For my car ('94 S500 and I suspect for yours), the rationale goes like this for the DIAGNOSTIC MODULE (N59) [yes, I know you do not have a DM]:
If a system malfunction occurs, the diagnostic module receives a malfunction code signal from the appropriate ECU. If the same malfunction code occurs during TWO (2) consecutive trips, the CHECK ENGINE light is illuminated and remains lit until the malfunction is illuminated and the DTC reset.
For some DTCs, the CE can go off if the same error does not appear for "X" trips. X is different for different malfunctions. A TRIP is defined as "engine temp above 80C, car moving for 5 minutes at more than 15mph" and a few other things I've forgotten.
But you get the idea. You can drive the car and if the error was a 'glitch', the CE light will reset itself. In my experience this does not happen often.
Regards . . . . JimF
'94 S500 Cpe
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