Cowls
These cowls on the IAE engines have a plunger that drops down when the cowls are opened this then weights a mechanism that holds the doors out a few inches so that the doors are either obviously open or obviously locked. To lock the cowl doors you would have to push the plunger up and lock the doors otherwise the plunger falls and the doors spring open a few inches again. This was a mod after a similar thing happened a few years ago - there was a picture on the net that a passenger took in flight with the cowl doors missing similar to the BA ones.
This means therefore at least one of the 6 laches was fastened and therefore the pilots would not have noticed anything strange on a walk around does it not?