@the hitcher; it's a normal thing at my workplace to have security guards doing walk arounds. They check for bombs etc, but I've had them come to me sometimes asking if some issues were normal... I can think about a dent caused by a loading truck AFTER I did my daily inspection. I think this is a good thing as long as you guards keep your hands off the airplane and don't open panels or play with switches.
@dream land; good thing you did spot it, and in main base they shouldn't have ADD'ed it. Paperwork for ADD takes more time nowadays than replacing the valva cap itself. However it's a non event, the cap is not the primary safety. You didn't know so best to ask the engineer indeed. I don't like the attitude of "I made the engineer" and "he was wrong" however... But alas, some pilots are like that.