The main and overload scans overlaps allowing the team to close them with a single bulkhead key. From there on, the team has all the time in the world to clean the main zones without worrying of the rising fog that starts after completing secondary objective. I doubt this was the developers' original idea for this mission. It could easily be fixed if one of the scans(either overload or main) is relocated and won't be exploited in future.
#ALT:R4D2 — Initial Bulkhead -scans overlapping allowing you to cheese the level.
nova hollow