Pax2908
Not necessarily! You have to understand that the normal flow, at least in my company, is that the walk round is conducted earlier rather than later so if there are any issues you can contact the engineers to get them rectified.
Basically, if the engineers are working on the engines you trust them to close the cowlings afterwards, it is that simple. Clearly, something did go wrong and we should leave it to the AAIB to determine the cause.
Prior to pushback the crew, captain in my company, asks the ground operative to confirm their checks and the reply should be "all doors and hatches closed etc".