If it was a service check or daily, Big Airways must use a task sheet for it and if the doors must be opened, then the paperwork should state it and the closing of as well, with stamper requirement to sign. I would be surprised if it wasn't. I cannot remember if having fan cowls open affects operation of the slats (doubt it). There is an AMM reference for the closing/opening the things..
I doubt another BA engineer looked at them (CAT A will have approvals for that task) and the final cheese hole was a flight crew member propably with his mind on another facet of the sector..