OP, where's the PCN pl?
Remember, the exceptions to No Stopping applied 10-4pm, therefore just being there at 4.30 was a contravention, what you were doing was immaterial. By the same token, there was no need for the CEO to observe for any minimum period because all they needed to know was that a car was stopped when prohibited and that none of the time-limited exceptions could apply.
What did you misunderstand about the sign?