BOAC - As I (SLF) read through the weekend's posts on this thread I wanted to raise the point you've made.
Quite a few times I've been on a plane where engineers are working on one of the engines (or at least one cowling has been up). Both pilot and co-pilot having been sitting up front -- the walk around presumably having been done.
The cowlings have been shut and off we go. There comes a point where you surely do have to trust the professionalism of others. Or are you meant to leap out and have a decko?
I assume baggage hold doors which are generally shut minutes or seconds before departure are not subject to being checked because there are warning lights in the cockpit to indicate that something is open.