i think it was pure confusion about not knowing fully what was going on, their were dummy conveys that were set by aircraft throwing out window at very specific times, that and the fog prolly obscured the bigger ships out to see. Also remeber it'd been a relatively quite coast (in terms of invasion threat) for 5 years or so before D-Day so the gaurds were prolly a bit of "o another boreing day on the guns again" mentalitly.