Seriously, folks - is it beyond Airbus to design a working latch that will hang down if it is not latched?
No, it's not and that's what they do; hang down if they are not latched.
I wonder if some bright (and in contrast with the outside colour) paint could be used over the full length/circumference, on the "edge" of the cowl which would not be normally visible when properly closed and latched...?
What time was the pilot's walkaround conducted at? Would they have been relying on artificial light? Sodium, perhaps. Ever look at dark blue in yellow light? I no longer fly but speaking as an industrial designer with a background in usability and human factors, the colour of the nacelles will be an issue that needs to be examined.