Interesting to note that the cowl door hold-open device (which was engaged) failed in its stated purpose ("to make it more obvious that the fan cowl doors are unlatched").
The AAIB seem in two minds about the effectiveness of said device:
"This accident, and numerous other fan cowl door loss events on A320 family aircraft, show that the gap is not sufficiently obvious to be detected reliably. The gap is obvious when pointed out, but not so in the absence of prior knowledge."
"[The co-pilot] stated that he was unaware of the fact that there is a visible gap between the fan cowl door and the nacelle when the fan cowl doors are unlatched and held open by the hold-open device. He stated, in hindsight, that if he had known this, he would have stood quarter-on to the engine to look at the fan cowl door shut lines and it would have been obvious to him on his external walk-around that the fan cowl doors were unlatched. However, he believed that this information had never been highlighted to him previously."